Optically active 3,3&#39;-dithiobis(2-amino-2methylpropionic acid) derivative and process for producing optically active 2-amino-3-mercapto-2-methylpropionic acid derivative

ABSTRACT

The present invention provides a useful novel intermediate and a novel synthetic process that can highly prevent contamination by various impurities to an optically active R or S isomer of a 2-amino-3-mercapto-2-methylpropionic acid derivative or salt thereof useful as an intermediate for pharmaceuticals and the like and provides a process for easily and efficiently producing a high purity optically active R or S isomer of a 2-amino-3-mercapto-2-methylpropionic acid derivative or salt thereof on an industrial production scale. A process of producing a high purity 2-amino-3-mercapto-2-methylpropionic acid derivative or salt thereof includes reductively cleaving a sulfur-sulfur bond of an intermediate, which is a high purity optically active 3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative substantially free of impurities. Thus, a resulting optically active 2-amino-3-mercapto-2-methylpropionic acid derivative can be produced without generating impurities as by-products which are difficult to remove.

TECHNICAL FIELD

The present invention relates to optically active R or S isomers of2-amino-3-mercapto-2-methylpropionic acid derivatives or salts thereof,which are useful as intermediates of medicines and the like, and toprocesses for producing optically active (2R,2′R) or(2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic acid) derivatives orsalts thereof, these compounds being useful as intermediates forpharmaceuticals and the like.

BACKGROUND ART

The processes for producing optically active R or S isomers of2-amino-3-mercapto-2-methylpropionic acid derivatives or salts thereofknown to heretofore include the following:

-   1) A process of asymmetric methylation to an optically active    thiazolidine compound obtained from optically active cysteine and    pivalaldehyde (WO01/72702);-   2) A process of asymmetric thiomethylation to an optically active    oxazolone compound obtained from optically active alanine and    benzaldehyde (J. Org. Chem., 1996, 61, 3350-3357);-   3) A process of asymmetric methylation to an oxazolone compound    obtained from cysteine and benzaldehyde (J. Org. Chem., 1992, 57,    5568-5573);-   4) A process of conducting asymmetric bromomethylation of an    optically active diketopiperazine compound obtained from optically    active valine and alanine and replacing the bromine atom of the    resulting compound by an alkali metal alkyl thiolate (Synthesis,    1983, 37-38);-   5) A process of synthesizing optically active aziridine from    optically active 2-methylglycidol obtained by Sharpless asymmetric    oxidation of 2-methyl-2-propen-1-ol, and then reacting the resultant    product with a thiol (J. Org. Chem., 1995, 60, 790-791); and-   6) A process of methylating an aminomalonic acid derivative,    conducting desymmetrization of the resulting product with porcine    liver esterase (referred to as “PLE” hereinafter), and reacting the    resulting unsymmetrical ester with a thioacetic acid alkali metal    salt (J. Am. Chem. Soc., 1993, 115, 8449-8450).

However, 2-amino-3-mercapto-2-methylpropionic acid derivatives or thesalts thereof have high water solubility and are difficult to extractwith organic solvents. In particular, unsubstituted2-amino-3-mercapto-2-methylpropionic acid or salts thereof havesignificantly high water solubility and cannot be extracted with organicsolvents regardless of the value of the pH. Thus, they are extremelydifficult to isolate. The only process for isolating the opticallyactive R or S isomer of the 2-amino-3-mercapto-2-methylpropionic acidderivative or salt thereof described is the one described in the process(WO01/72702) under item 1) above, the process in which the aqueoushydrochloric acid solution of optically active2-amino-3-mercapto-2-methylpropionic acid is concentrated to solidify asthe hydrochloride thereof.

In each of the processes described in items 1) to 6) above, anappropriate auxiliary group is introduced to the starting materials usedin stereoselective reaction in order to attain high stereoselectivityand to thereby obtain an optically active compound having a targetconfiguration. Thus, in order to obtain the target optically active2-amino-3-mercapto-2-methylpropionic acid derivative or salt thereof,the auxiliary group must be removed from the optically activeintermediate obtained by the stereoselective reaction, i.e., theoptically active 2-amino-3-mercapto-2-methylpropionic acid derivative orsalt thereof containing the auxiliary group. Here, the term “auxiliarygroup” refers to a substituent having an effect of improving thestereoselectivity in the course of the stereoselective reaction, or asubstituent (known as “protecting group”) that has an effect ofpreventing the undesirable action of a functional group that has aneffect of inhibiting the reaction.

In the course of removing the auxiliary group from the optically activeintermediate to produce an optically active2-amino-3-mercapto-2-methylpropionic acid derivative or salt thereof,that an organic or an inorganic substance derived from the auxiliarygroup or a reagent used for removing the auxiliary group is generated asan impurity, i.e., a by-product. In some cases, an inorganic substancemay be produced as the impurity, i.e., the by-product, in post-reactiontreatment such as neutralization. Among these impurities, water-solubleimpurities, such as inorganic substances, are extremely difficult toseparate from the optically active 2-amino-3-mercapto-2-methylpropionicacid derivative having high water solubility. Thus, the optically active2-amino-3-mercapto-2-methylpropionic acid derivative or salt thereof issubjected to the isolation process described in the process stated inthe item 1) above while it still contains these water-solubleimpurities. As a result, the water-soluble impurities exist togetherwith the optically active 2-amino-3-mercapto-2-methylpropionic acidhydrochloride, and there is a tendency of the impurities contaminatingthe product optically active 2-amino-3-mercapto-2-methylpropionic acidhydrochloride. In the description below, an example of removing benzylgroup, which is a thioether auxiliary group on the sulfur atom ofoptically active 2-amino-3-benzylthio-2-methylpropionic acid obtained bythe process described in item 3) above, so as to produce opticallyactive 2-amino-3-mercapto-2-methylpropionic acid is described.

In general, the benzyl thioether auxiliary group is removed by a process(known as Birch reduction) that uses metallic sodium and liquid ammonia.In this process, as the post-reaction treatment, unreacted metallicsodium is decomposed with alcohol or water and then the reductionproducts are extracted with an organic solvent to remove water-solublesodium compounds, such as sodium hydroxide. However, when this processis applied to optically active 2-amino-3-benzylthio-2-methylpropionicacid, the resultant product, namely,2-amino-3-mercapto-2-methylpropionic acid, cannot be extracted with anorganic solvent regardless of the value of the pH and it has beennecessary to add hydrochloric acid to the product still containingsodium compounds, such as sodium hydroxide, and then conduct theisolation process described in the process set forth in the item 1)above. As a result, it has been found that sodium chloride and the likegenerated between the sodium compound and the hydrochloric acid addedsolidify together with the optically active2-amino-3-mercapto-2-methylpropionic acid hydrochloride, therebycontaminating the product optically active2-amino-3-mercapto-2-methylpropionic acid hydrochloride.

The present inventors have also found that optically active2-amino-3-mercapto-2-methylpropionic acid is relatively unstable, thatimpurities possibly derived from optically active2-amino-3-mercapto-2-methylpropionic acid are readily produced as theby-products, that these impurities once generated are difficult toremove, and that it is difficult to prevent these impurities fromcontaminating the product. It is conceivable that impurities derivedfrom optically active 2-amino-3-mercapto-2-methylpropionic acid arestructural analogs that have parts similar to those of2-amino-3-mercapto-2-methylpropionic acid. As is commonly known,structural analogs having a structure similar to the target compoundbehave like the target compound through the operational steps from thereaction to the post treatment and thus are easily mixed into the finalproducts. In particular, in a case where the final product ispharmaceuticals, mixing of even a trace amount of an impurity mayinflict a serious problem. Thus, it is vital that a process that canhighly suppress generation of impurities and contamination beestablished.

As is discussed above, in the related art, no process that can suppresscontamination by the above-described various impurities has beenestablished, and there have been problems in the industrial process ofproducing the optically active 2-amino-3-mercapto-2-methylpropionic acidderivatives or salts thereof. Thus, establishment of an industriallypracticable process for producing high-quality optically active2-amino-3-mercapto-2-methylpropionic acid derivatives and salts thereofhas been strongly desired.

DISCLOSURE OF INVENTION

Under these circumstances, the present invention aims to develop auseful novel intermediate and a novel synthetic process that can highlyprevent contamination by various impurities in the process of producingan optically active R or S isomer of a2-amino-3-mercapto-2-methylpropionic acid derivative or salt thereofuseful as an intermediate for pharmaceuticals and the like, and toprovide a process for easily and efficiently producing a high purityoptically active R or S isomer of a 2-amino-3-mercapto-2-methylpropionicacid derivative or salt thereof on an industrial production scale.

In aiming to achieve the object, the present inventors have vigorouslyinvestigated the process that can highly suppress contamination to theoptically active 2-amino-3-mercapto-2-methylpropionic acid derivative orsalt thereof by the above-described various impurities, namely,impurities derived from the auxiliary group, impurities derived from thereagent used to remove the auxiliary group, inorganic substancesproduced as by-products in the post-reaction treatment, such asneutralization, and impurities derived from the optically active2-amino-3-mercapto-2-methylpropionic acid derivative. As a result, theinventors have found that an optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative in which theauxiliary group (the protecting group) on the sulfur atom is asymmetrical disulfide protecting group can serve as an excellentintermediate that solves the above-described problems. In other words,the present inventors have found that a high purity optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative can be easilyobtained by removing impurities and that the resulting optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative can bequantitatively converted to a corresponding optically active2-amino-3-mercapto-2-methylpropionic acid derivative (target substance)by reductive cleavage of the sulfur-sulfur bond of the optically active3, 3′-dithiobis(2-amino-2-methylpropionic acid) derivative withoutgenerating by-products, i.e., impurities, which are difficult to remove.

The inventors have also found that, with this process, the contaminationto the product optically active 2-amino-3-mercapto-2-methylpropionicacid derivative by the various impurities, which has been difficult toavoid according to the conventional process, can be minimized, and thatthe amounts of the impurities derived from the optically active2-amino-3-mercapto-2-methylpropionic acid derivative can also beminimized. As is stated above, suppression of the contamination to theproduct optically active 2-amino-3-mercapto-2-methylpropionic acidderivative by various by-products, which have been difficult to removeby the conventional process, can be easily and efficiently achievedthrough the use of the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative.

The important intermediate of the present invention, i.e., the 3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative described above,has three optical isomers, namely, a (2R,2′R) isomer, a (2S,2′S) isomer,and a meso isomer ((2R,2′S) or (2S,2′R)) The target optical isomers ofthe present invention are the (2R,2′R) isomer and the (2S,2′S) isomer.

In particular, the present invention relates to a process for producingan optically active 2-amino-3-mercapto-2-methylpropionic acid derivative(1) represented by general formula (1) below or salt thereof:

(wherein Y¹ is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z¹ is a substituted or unsubstituted aminogroup, or Y¹ and Z¹ together form a divalent group; and * is anasymmetric carbon), the process including:

reducing an optically active 3,3′-dithiobis(2-amino-2-methylpropionicacid) derivative represented by formula (2) below or salt thereof:

(wherein Y² and Z² respectively may be the same as or different from Y¹and Z¹ above; Y² is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z² is a substituted or unsubstituted aminogroup, or Y² and Z² together form a divalent group; and * is anasymmetric carbon) so as to cleave the sulfur-sulfur bond; andconverting Y² to Y¹ and/or Z² to Z¹ as necessary.

The present invention also relates to a process for producing theoptically active 3,3′-dithiobis(2-amino-2-methylpropionic acid)represented by said general formula (2) or salt thereof, the processincluding oxidizing an optically active 3-mercapto-2-methylpropionicacid derivative represented by general formula (3) below or salt thereof

(wherein Y³ and Z³ may respectively be the same as or different from Y²and Z² above; Y³ is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z³ is a substituted or unsubstituted aminogroup, or Y³ and Z³ together form a divalent group; and * is anasymmetric carbon) to form a sulfur-sulfur bond between two molecules;and converting Y³ to Y² and/or Z³ to Z² as necessary.

The present invention also relates to a process for purifying anoptically active 3,3′-dithiobis(2-amino-2-methylpropionic acid)derivative represented by said general formula (2) or salt thereof, theprocess including adjusting an aqueous medium solution containing theoptically active 3,3′-dithiobis(2-amino-2-methylpropionic acid)derivative represented by said general formula (2) or salt thereof to bebasic to separate and remove impurities from the solution.

The present invention also relates to a process for purifying theoptically active 3,3′-dithiobis(2-amino-2-methylpropionic acid)derivative (2), the process including adjusting an aqueous mediumsolution containing the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative represented bysaid general formula (2) or salt thereof to be neutral to acidic so asto crystallize the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative (2), andremoving impurities.

The present invention also relates to a process for purifying a saltwith an acid of the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative represented bysaid general formula (2), the process including adjusting an aqueousmedium solution containing the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative represented bysaid general formula (2) or salt thereof to be highly acidic so as tocrystallize the salt with the acid of the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative (2), andremoving impurities.

The present invention also relates to an optically active (2R,2′R) or(2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic acid) derivativerepresented by formula (4) below or salt thereof:

(wherein —Y⁴-Z⁴- is adivalent group; and * is an asymmetric carbon)

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in detail. First, thecompounds of the present invention are described.

A 2-amino-3-mercapto-2-methylpropionic acid derivative represented bysaid general formula (1) is described first. Hereinafter, thisderivative is also referred to as “compound (1)”. In the compound (1), *indicates an asymmetric carbon, Y¹ and Z¹ may each independently be amonovalent group or may together form a divalent group. When Y¹ and Z¹are each independently a monovalent group, Y¹ is an unsubstitutedhydroxyl group or a substituted or unsubstituted amino group and Z¹ is asubstituted or unsubstituted amino group.

When the Y¹ and Z¹ are each independently a monovalent group, inparticular, examples of the substituent in the substituted amino groupinclude aminocarbonyl group having 1 to 20 carbon atoms, alkoxycarbonylgroup having 2 to 20 carbon atoms, acyl group having 1 to 20 carbonatoms, and a monovalent organic group having 1 to 20 carbon atoms. Thesesubstituents may be substituted or unsubstituted. The substituted aminogroup may be monosubstituted or disubstituted. When the amino group isdisubstituted, any combination of the above-described substituents canbe used. Examples of the aminocarbonyl group having 1 to 20 carbon atomsinclude methylaminocarbonyl group, ethylaminocarbonyl group, andbenzylaminocarbonyl group. Examples of the alkoxycarbonyl group having 1to 20 carbon atoms are carbamate protecting groups for amino groups,such as methoxycarbonyl group, ethoxycarbonyl group, tert-butoxycarbonylgroup, and benzyloxycarbonyl group. Examples of the acyl group having 1to 20 carbon atoms are amide-type or imide-type protecting groups foramino groups and include formyl group, acetyl group, benzoyl group, andphthaloyl group. Examples of the monovalent organic group having 1 to 20carbon atoms include alkyl groups having 1 to 20 carbon atoms, such asmethyl group, ethyl group, n-propyl group, isopropyl group,n-butylgroup, isobutylgroup, sec-butylgroup, tert-butylgroup, n-pentylgroup, isopentyl group, and n-hexyl group; aralkyl groups having 7 to 20carbon atoms, such as benzyl group, 4-methylbenzyl group, 3-methylbenzylgroup, 2-methylbenzyl group, 4-methoxybenzyl group, 3-methoxybenzylgroup, 2-methoxybenzyl group, 1-phenylethyl group, 2-phenylethyl group,1-(4-methylphenyl)ethyl group, 1-(4-methoxyphenyl)ethyl group,3-phenylpropyl group, and 2-phenylpropyl group; and aryl groups having 6to 20 carbon atoms, such as phenyl group, 1-naphthyl group, 2-naphthylgroup, 4-methylphenyl group, 3-methylphenyl group, 2-methylphenyl group,4-ethylphenyl group, 3-ethylphenyl group, 4-methoxyphenyl group,3-methoxyphenyl group, 2-methoxyphenyl group, 4-nitrophenyl group,4-phenylphenyl group, 4-chlorophenyl group, and 4-bromophenyl group.

The substituent contained in the substituted amino group may have afunctional group as long as the essence of the present invention is notimpaired (i.e., as long as the series of reactions is not particularlyadversely affected). Examples of the functional group include aminogroup, hydroxyl group, phenyl group, aryl group, alkanoyl group, alkylgroup, alkenyl group, alkynyl group, alkoxyl group, nitro group, andhalogen atom.

When Y¹ and Z¹ are each independently a monovalent group, it ispreferable that Y¹ and Z¹ are respectively an unsubstituted hydroxylgroup and a substituted or unsubstituted amino group or are respectivelyan unsubstituted hydroxyl group and a substituted or unsubstitutedureido group (—NHCONH₂). More preferably, Y¹ and Z¹ are respectively anunsubstituted hydroxyl group and an unsubstituted amino group orrespectively an unsubstituted hydroxyl group and an unsubstituted ureidogroup. Most preferably, Y¹ and Z¹ are respectively an unsubstitutedhydroxyl group and an unsubstituted amino group.

When Y¹ and Z¹ together form a divalent group, Y¹ represents asubstituted hydroxyl group or a substituted amino group, and Z¹represents a substituted amino group. In other words, in this divalentgroup, the terminus at the Y¹ side is either an oxygen atom or anitrogen atom, and the terminus at the Z¹ side is a nitrogen atom. Noparticular limitation is imposed other than the Y¹-side terminus (oxygenor nitrogen atom) and the Z¹-side terminus (nitrogen atom); however, thedivalent group preferably forms a five-membered ring or six-memberedring by incorporating into the structure of the compound (1).

Specific examples of the divalent group (represented by —Y¹-Z¹-) includea substituted or unsubstituted-ureylene group (—NHCONH—), a substitutedor unsubstituted 1-oxa-3-aza-2-propanone-1,3-diyl group (—OCONH—), asubstituted or unsubstituted 1-oxa-3-aza-propane-1,3-diyl group(—OCH₂NH—), a substituted or unsubstituted1-oxa-3-aza-2-propene-1,3-diyl group (—OCH═N—), and a substituted orunsubstituted 1,4-diaza-2-butanone-1,4-diyl group (—NHCH₂CONH—).Examples of the substituent of the divalent group include aminocarbonylgroup having 1 to 20 carbon atoms, alkoxycarbonyl group having 1 to 20carbon atoms, acyl group having 1 to 20 carbon atoms, and a monovalentorganic group having 1 to 20 carbon atoms.

The substituent of the divalent group may have a functional group aslong as the series of the reactions of the present invention is notparticularly adversely affected. Examples of the functional groupinclude amino group, hydroxyl group, phenyl group, aryl group, alkanoylgroup, alkyl group, alkenyl group, alkynyl group, alkoxyl group, nitrogroup, and halogen atom.

When the Y¹ and Z¹ together form a divalent group, it is preferable thatY¹ and Z¹ together form a substituted or unsubstituted ureylene group(—NHCONH—); and in particular, an unsubstituted ureylene group.

Next, the 3,3′-dithiobis(2-amino-2-methylpropionic acid) derivativerepresented by said general formula (2) (hereinafter this derivative isalso referred to as “compound (2)”) is described. In the compound (2), *is the same as above, and Y² and Z² may each independently be amonovalent group, may together form a divalent group, or may be the sameas Y¹ and Z¹ described above. When Y² and Z² are each independently amonovalent group, Y² is an unsubstituted hydroxyl group or a substitutedor unsubstituted amino group and Z² is a substituted or unsubstitutedamino group.

The specific examples of Y² and Z² are the same as those described asthe examples of Y¹ and Z¹.

When Y² and Z² are each independently a monovalent group, Y² and Z² arepreferably respectively an unsubstituted hydroxyl group and asubstituted or unsubstituted amino group, or an unsubstituted hydroxylgroup and a substituted or unsubstituted ureido group (—NHCONH₂). Morepreferably, Y² and Z² are respectively an unsubstituted hydroxyl groupand an unsubstituted amino group, or an unsubstituted hydroxyl group andan unsubstituted ureido group. Most preferably, Y² and Z² arerespectively an unsubstituted hydroxyl group and an unsubstituted aminogroup.

When Y² and Z² together form a divalent group, it is preferable that Y²and Z² together form a substituted or unsubstituted ureylene group(—NHCONH—) and in particular, an unsubstituted ureylene group.

Thirdly, the 3-mercapto-2-methylpropionic acid derivative represented bysaid general formula (3) (hereinafter this derivative is also referredto as “compound (3)”) is described. In the compound (3), * is the sameas above; and Y³ and Z³ may each independently be a monovalent group,may together from a divalent group, or may be the same as Y² and Z²described above. When Y³ and Z³ are each independently a monovalentgroup, Y³ is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group, and Z³ is a substituted or unsubstitutedamino group.

Specific examples of Y³ and Z³ are the same as those described as theexamples of Y¹ and Z¹.

When Y³ and Z³ are each independently a monovalent group, Y³ and Z³ arepreferably an unsubstituted hydroxyl group and a substituted orunsubstituted amino group, respectively, or an unsubstituted hydroxylgroup and a substituted or unsubstituted ureido group (—NHCONH₂),respectively. More preferably, Y³ and Z³ are an unsubstituted hydroxylgroup and an unsubstituted amino group, respectively, or anunsubstituted hydroxyl group and an unsubstituted ureido group,respectively. Most preferably, Y³ and Z³ are an unsubstituted hydroxylgroup and an unsubstituted amino group, respectively.

When Y³ and Z³ together form a divalent group, it is preferable that Y³and Z³ together form a substituted or unsubstituted ureylene group(—NHCONH—), and in particular, an unsubstituted ureylene group.

According to the present invention, an effect of removing variousby-products can be expected as described in detail below. Thus, thecompound (3) containing various impurities, such as by-products, e.g.,inorganic substances generated in the course of production of thecompounds, and impurities derived from the compounds, may be usedwithout any problem. Rather, the present invention is truly effectivewhen the compound (3) containing impurities is used.

Fourthly, the optically active (2R,2′R) or(2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic acid) derivativerepresented by said general formula (4) (hereinafter, this derivative isalso referred to as “compound (4)”) or salt thereof is described. Thecompound (4) is a useful compound as an intermediate for pharmaceuticalsdiscovered in the present invention.

Both the (2R,2′R) optical isomer and the (2S,2′S) optical isomer of thecompound (4) are included in the scope of the present invention.

In the compound (4), * is the same as above, and —Y⁴-Z⁴- preferablyrepresents a divalent group that forms a five- or six-membered ring byincorporating into the structure of the compound (4). Specific examplesof the divalent group include a substituted or unsubstituted ureylenegroup (—NHCONH—), a substituted or unsubstituted1-oxa-3-aza-2-propanone-1,3-diyl group (—OCONH—), a substituted orunsubstituted 1-oxa-3-aza-propane-1,3-diyl group (—OCH₂NH—), asubstituted or unsubstituted 1-oxa-3-aza-2-propene-1,3-diyl group(—OCH═N—), and a substituted or unsubstituted1,4-diaza-2-butanone-1,4-diyl group (—NHCH₂CONH—). Examples of thesubstituent of the divalent group above include the aminocarbonyl grouphaving 1 to 20 carbon atoms, the alkoxycarbonyl group having 2 to 20carbon atoms, the acyl group having 1 to 20 carbon atoms, and themonovalent organic group having 1 to 20 carbon atoms mentioned above.

The substituent in the divalent group above may have a functional groupas long as the series of reactions is not particularly adverselyaffected. Examples of the functional group include amino group, hydroxylgroup, phenyl group, aryl group, alkanoyl group, alkyl group, alkenylgroup, alkynyl group, alkoxyl group, nitro group, and halogen atom.

The group —Y⁴-Z⁴- is more preferably a substituted or unsubstitutedureylene group (—NHCONH—), and most preferably an unsubstituted ureylenegroup (—NHCONH—). An optically active (2R,2′R) or(2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative,(a.k.a. (5R,5′R) or(5S,5′S)-5,5-[dithiobis(methylene)]bis(5-methylhydantoin) derivative)represented by formula (5) below is preferred as the compound (4):

The individual reaction steps will now be described in detail.

A process of producing the 3,3′-dithiobis(2-amino-2-methylpropionicacid) derivative (2) or salt thereof used for the reduction in thepresent invention is described first.

The process of preparing the compound (2) is not particularly limitedand any method can be used without any limitation. Among theseprocesses, a process of forming a sulfur-sulfur bond between themolecules of the 3-mercapto-2-methylpropionic acid derivative (3) orsalt thereof and converting Y³ to Y² and/or Z³ to Z² as necessary ispreferable. Here, the meaning of the phrase “to convert Y³ to Y² and/orZ³ to Z²” is, for example, to convert the substituted amino group toanother substituted amino group by derivatization of the substituent, toconvert the substituted amino group to an unsubstituted amino group byremoval of the substituent, or to convert the unsubstituted amino groupto a substituted amino group by introduction of a substituent. When Y³and Z³ together form a divalent group, this phrase means that, forexample, the divalent group is converted to another divalent group byderivatization or to a monovalent group indicated by Y² and Z² above.When conversion of Y³ to Y² and/or Z³ to Z² is performed, the conversionmay take place after forming the sulfur-sulfur bond between themolecules of the compound (3) or salt thereof or before the formation ofthe sulfur-sulfur bond. The conversion of Y³ to Y² and/or Z³ to Z² maybe conducted simultaneously with the formation of the sulfur-sulfurbond.

Examples of the process for forming the sulfur-sulfur bond between themolecules of the 3-mercapto-2-methylpropionic acid derivative (3) orsalt thereof and converting Y³ to Y² and/or Z³ to Z² as necessaryinclude a process for producing an optically active3,3′-dithiobis(2-methylpropinionic acid) represented by formula (2a)below or salt thereof:

(which is a compound represented by general formula (2) above with Y²representing an unsubstituted hydroxyl group and Z² representing anunsubstituted amino group, hereinafter, this compound is also referredto as the “compound (2a)”), the process including forming asulfur-sulfur bond between molecules of5-mercaptomethyl-5-methylhydantoin represented by formula (3b) below orsalt thereof:

(which is a compound represented by general formula (3) above with Y³and Z³ together forming ureylene group (—NHCONH—), this compound ishereinafter also referred to as the “compound (3b)”) to convert thecompound (3b) to optically active 5,5′-[dithiobis(methylene)]bis(5-methylhydantoin) represented by formula(2′b) below or salt thereof:

(this compound is hereinafter also referred to as the “compound (2′b)”),and then cleaving the hydantoin ring by hydrolysis of the ureylene groupto convert the compound (2′b) to optically active3,3′-dithiobis(2-methylpropionic acid) (2a) or salt thereof.Alternatively, the compound (2a) or salt thereof may be prepared byforming a sulfur-sulfur bond between molecules of2-amino-3-mercapto-2-methylpropionic acid represented by formula (3a)below or salt thereof:

(which is a compound represented by general formula (3) above with Y³representing an unsubstituted hydroxyl group and Z³ representing anunsubstituted amino group, this compound is hereinafter also referred toas the “compound (3a)”).

The compound (3a) and salt thereof described above may be obtained byhydrolysis of the ureylene group of the compound (3b) or salt thereof.Embodiments of producing the compound (2a) from the compound (3b)through the compound (3a) are also included in the scope of the presentinvention.

The process of forming a sulfur-sulfur bond between molecules of thecompound (3) or salt thereof will now be described.

Examples of the process for forming the intermolecular sulfur-sulfurbond include a process involving reductive reaction of a halogenatedsulfonyl compound, a halogenated sulfenyl compound, or a sulfinic acidcompound; a process involving a reaction accompanied by cleavage of thecyano group of the thiocyanate compound; and a process involvingoxidation of a thiol compound. The oxidation of a thiol compound easilyand efficiently produces a high purity compound (2) or salt thereof andis thus particularly preferable. A process of producing a compound (2)or salt thereof by forming an intermolecular sulfur-sulfur bond by theoxidation of the compound (3) or salt thereof will now be described.

The oxidant used in the oxidation of the thiol compound is notparticularly limited. Various oxidants may be used including oxygen(air); aqueous hydrogen peroxide; halogens such as bromine and iodine;hypohalous acids such as hypochlorous acid; sulfoxides such asdimethylsulfoxide; and transition metals such as manganese(IV) oxide,iron(III) chloride, and potassium hexacyanoferrate(III). The amount ofthe oxidant used cannot be categorically determined since it differsdepending on the type of the oxidant. Preferably, a stoichiometricamount or more of the oxidant relative to the compound (3) is used fromthe standpoints of increasing the reaction rate and the yield.

Preferable examples of the oxidant are oxygen, aqueous hydrogenperoxide, halogens, and hypohalous acids. Among these, oxygen isparticularly preferable since oxygen used as the oxidant can easily andefficiently produce a high-quality compound (2) or salt thereof byhighly suppressing the side reaction. The oxidation usually has poorselectivity and the side reaction is sometimes difficult to control. Inthe oxidation of thiol compounds, for example, sulfonic acid and thelike may be generated as the by-product, or excessive oxidation to thesulfoxides (thiosulfinate) may even occur. When oxygen is used as theoxidant, i.e., the oxygen oxidation or air oxidation, the oxidizingpower is small and a practicable level of reaction rate may not beobtained. However, the investigation conducted by the present inventorsshows that, contrary to the expectation, the compound (3) or saltthereof can be selectively converted to the target compound (2) or saltthereof, and that the conversion to the compound (2) or salt thereof canbe carried out at a practical reaction rate even by the oxygen oxidationusually known to have small oxidation power. The details are describedbelow by taking an example of oxidation by oxygen oxidation.

The process of introducing oxygen used as the oxidant is notparticularly limited. Compressed air as an oxygen-containing gas may beintroduced to a reactor using a steel cylinder or compressor. Naturally,oxygen may be directly fed or may be introduced as a mixed gas in whichoxygen is diluted with an inert gas such as nitrogen, by using a steelcylinder of liquefied oxygen. In this case, it is possible to adjust theoxygen concentration in the reactor or in the mixed gas to be introducedso as to adequately control the amount of dissolved oxygen in thereaction solution.

In order to supply oxygen, the oxygen-containing gas (oxygen, compressedair, or mixed gas) may be introduced in the gas phase or liquid phase ofthe reactor while evacuating from the vent (also known as aerationcondition), or the internal pressure of the reactor may be adjusted to apredetermined pressure (not particularly limited but preferably normalpressure or high pressure) by supplying oxygen in an amountcorresponding to the partial pressure consumed by the oxidation asnecessary.

When oxygen is supplied by aeration, the reaction rate is greatlyaffected by the method of introducing the oxygen-containing gas (oxygen,compressed air, or mixed gas). From the standpoint of oxygen supplyefficiency, a method of introducing the gas to the liquid phase is moreefficient than the method of introducing the gas to the gas phase. Theoxygen supply efficiency (oxidation rate) is also greatly affected bythe manner of introduction. In order to efficiently supply oxygen, it isimportant that the contact efficiency between the oxygen-containing gasand the reaction solution be increased. In order to increase the contactefficiency with the reaction solution, it is preferable to break upbubbles generated by the introduction of the oxygen-containing air tothe liquid phase as much as possible and to disperse the resultingbroken bubbles so that the surface area of the bubbles is increased,thereby promoting migration of oxygen. Thus, in the case of an aerationnozzle having a single tube provided with a tip inserted into thereaction solution, the tip preferably includes a plurality of smallholes. Moreover, it is particularly preferable to install aperforated-pipe distributor (sparger) at the bottom of the reactor belowthe stirring blade since the bubbles are broken up by the stirring bladeduring the course of rising from the bottom of the reactor.

With respect to the stirring during the reaction, excessively weakstirring tends to result in trapping of the gas (oxygen) at thegas-liquid interface at the surface of the reaction solution and ininsufficient dispersion of the bubbles in the solution. Thus, in orderto efficiently supply oxygen and to rapidly proceed the oxidation, amiddle or higher level of stirring is preferable, in particular, thestirring is preferably as vigorous as possible. The stirring is notlimited to stirring by blades, and it is effective to circulate thereaction solution. Examples of the method for circulation include amethod (internal circulation inside the reactor) for generating upwardflow and downward flow of the reaction solution by providing a partitionor a baffle plate inside the reactor; and a method (externalcirculation) in which the reaction solution is circulated by a flow pathdisposed outside the reactor.

In the course of oxygen oxidation, there is a tendency that the reactionis promoted by basifying the reaction solution. In order to basify thereaction solution, the-oxidation may be conducted in the presence of abasic substance or a basic substance may be added to the reactionsolution. Although the basic substance is not particularly limited, useof inorganic bases is preferred. Examples of the inorganic bases includealkali metal hydroxides such as lithium hydroxide and sodium hydroxide;alkali metal carbonates such as sodium carbonate and potassiumcarbonate; and alkali metal hydrogen carbonates such as sodium hydrogencarbonate and potassium hydrogen carbonate.

The amount of the basic substance used is not particularly limited. Forexample, the molar amount of the basic substance is 0.1 to 10 times,preferably 0.5 to 5 times, and most preferably 0.5 to 2 times the molaramount of the compound (3). Note that when the basicity of the reactionsolution to which the basic substance is added is indicated in terms ofthe pH, the pH is usually 8 or more, preferably 9 or more, and mostpreferably 10 or more. At an excessively high pH, side reaction such asdecomposition of the compound (3) or compound (2) tends to occur. Inorder to inhibit the side reaction, the pH is usually 14 or less,preferably 13 or less, and most preferably 12 or less.

The reaction solvent is not particularly limited, and various solventsare usable. One type of solvent may be used alone as the reactionsolvent, or a mixture of two or more types of solvent may be used. Whenan inorganic base is used as the basic substance, it is particularlypreferable to use, as the reaction solvent, water alone or a mixture ofwater and another solvent.

The temperature of the reaction is not particularly limited but shouldbe higher than the temperature (solidification point) at which thereaction solution does not solidify. The temperature is usually −10° C.or higher, preferably 0° C. or higher, and most preferably 10° C. orhigher. From the standpoint of increasing the reaction rate, it ispreferable to increase the reaction temperature. The upper limit of thereaction temperature is not particularly limited but should not exceedthe boiling point of the reaction solution. As a matter of course, it ispossible to conduct reaction under the reflux conditions. Since thesaturated concentration of oxygen in the reaction solution decreaseswith the increasing temperature, it is possible that the reaction rateis decreased at an excessively high temperature. The reactiontemperature should be set to an appropriate level by taking intoconsideration these effects. The reaction temperature is usually 100° C.or lower, preferably 80° C. or lower, and most preferably 60° C. orlower.

In the course of the oxygen oxidation, the reaction may be performed inthe presence of an adequate amount of an oxidation catalyst or by addingan adequate amount of an oxidation catalyst so as to accelerate thereaction. In this manner, the reaction can be carried out at a lowertemperature at a high rate; thus, the oxidation can be conducted at amoderate process temperature. The process temperature in such a case isusually 40° C. or less, furthermore a practicable reaction rate can bemaintained even at a temperature of 25° C. or less.

The oxidation catalyst may be any catalyst that has an effect ofpromoting the reaction. Examples thereof are ionic compounds of heavymetals (transition metals) including divalent ionic iron compounds suchas iron(II) chloride and iron(II) hydroxide and trivalent ionic ironcompounds such as iron(III) chloride and iron(III) hydroxide; cupriccompounds such as copper(II) sulfate and copper(II) hydroxide; andcobalt complexes such as phthalocyanine cobalt. Among these, ionic ironcompounds and cupric compounds are particularly preferred since they areeasy to remove in the post-treatment described below. Ionic ironcompounds are yet more preferable.

By the oxidation, an intermolecular sulfur-sulfur bond of the compound(3) or salt thereof can be formed at a high reaction conversion ratio. Aconversion ratio of at least 95%, usually 98% or more, preferably 99% ormore, and more preferably 99.9% or more can be expected.

As a result of the oxidation and the like, an intermolecularsulfur-sulfur bond can be formed between the molecules of the compound(3) or salt thereof. The Y³ and Y² and/or Z³ and Z² may be the same ordifferent. When they are different, conversion of Y³ to Y² and/or Z³ toZ² is necessary to produce 3,3′-dithiobis(2-amino-2-methylpropionicacid) derivative (2) or salt thereof. Reaction for converting Y³ to Y²and/or Z³ to Z² may be conducted before or after the formation of theintermolecular sulfur-sulfur bond or simultaneously with the formationof the intermolecular sulfur-sulfur bond. The method for converting Y³to Y² and/or Z³ to Z² will be described below.

The method for converting Y³ to Y² and/or Z³ to Z² is not particularlylimited. For example, a method described in Protective Groups in OrganicSynthesis, 2nd ed., John Willy & Sons (1991) may be used. The suitablemethod differs depending on the types of Y³ and Z³. Examples of themethod include acid treatment, base treatment, hydrolysis, nitrous acidoxidation, and Na/NH₃ treatment. From the standpoint of operability,hydrolysis is preferred and hydrolysis using an acid is particularlypreferred. An example of forming a sulfur-sulfur bond between twomolecules of the compound (3b) or salt thereof to produce the compound(2′b) or salt thereof and then converting the resulting compound to thecompound (2a) or salt thereof is described below.

First, in the manner described above, a sulfur-sulfur bond is formedbetween two molecules of the compound (3b) or salt thereof to producethe compound (2′b). In the case where the compound (2′b) or salt thereofis converted to the compound (2a) or salt thereof, the ureylene group(—NHCONH—, —Y³-Z³-) should be converted to the unsubstituted hydroxylgroup (Y²) and the unsubstituted amino group (Z²). In particular, thecompound (2′b) or salt thereof may be hydrolized under acidic conditionsusing hydrochloric acid or the like to cleave the hydantoin ring tothereby produce the compound (2a) or salt thereof.

In this manner, the compound (2) or salt thereof can be easily andefficiently produced by forming a sulfur-sulfur bond between moleculesof the compound (3) or salt thereof and converting Y³ to Y² and/or Z³ toZ² as necessary.

Next, the method for purifying and isolating the compound (2) or saltthereof is described. The purification and isolation of the compound (2)is an important procedure of the present invention for preliminarilyeliminating impurities or precursor thereof, which are difficult toremove once they become mixed with the target compound (1), at the stageof forming the compound (2). The impurities may be derived from anysubstance, and examples thereof include a reagent, such as an oxidationcatalyst, used in the step of producing the compound (2) and derivativestherefrom, and substances contained in the starting materials, such asthe compound (3), in trace amounts. The present inventors have searchedfor an efficient isolation and purification process that cansatisfactorily remove the impurities and consequently found that thecompound (2) and the impurities, in particular, the inorganic substancesthat are difficult to remove once they are mixed in the compound (1),can be efficiently separated by adequately adjusting the acidity orbasicity of an aqueous medium solution containing the compound (2).Thus, the present inventors have established the process for purifyingand isolating the compound (2). Examples of the inorganic substancesinclude ionic compounds of representative metals, e.g., alkali metals,such as lithium, sodium, and potassium, and ionic compounds of heavymetals (transition metals) such as iron, copper, and cobalt that serveas the oxidation catalysts. In particular, the purification process ofthe present invention can efficiently separate the catalytic heavy metalionic compounds which are no longer necessary after the oxidation, inparticular, ionic iron compounds. The purification and isolation processcan be roughly classified into the following three types:

-   1) A process for purifying a salt with a base of the compound (2),    in which the aqueous medium solution is adjusted to be basic;-   2) A process for purifying the compound (2), in which the aqueous    medium solution is adjusted to be neutral to acidic; and-   3) A process for purifying a salt with an acid of the compound (2),    in which the aqueous medium solution is adjusted to be highly    acidic.

The first purification process, in which the salt with the base of thecompound (2) is purified by adjusting the aqueous medium solutioncontaining the compound (2) or salt thereof to be basic so as toseparate and remove the impurities from the solution, is describedfirst.

According to the first purification process, an aqueous medium solutioncontaining the compound (2) or salt thereof is adjusted to be basic sothat the salt with the base of the compound (2) is dissolved in theaqueous medium and that impurities sparingly soluble in the aqueousmedium are separated from the solution. In order to adjust the solutionto be basic, the pH of the solution may be used as an index. The lowerlimit of the pH is usually 8 or more, preferably 9 or more, and mostpreferably 10 or more from the standpoint of removing the impurities.The basic substance added to adjust the solution to be basic is notparticularly limited. Any commercially available organic base and/orinorganic base may be used.

The impurity may be precipitated from the aqueous medium as a solid orseparated as liquid depending on the type of the impurity. When theimpurity is precipitated as a solid from the aqueous medium, theprecipitated solid impurity can be separated and removed by a typicalsolid-liquid separation such as pressure filtration, vacuum filtration,or centrifugal filtration. For example, when the impurity is an ioniciron compound, such as iron(III) hydroxide, the impurity has a lowsolubility under basic conditions and can be precipitated as a solid.However, the ionic iron compound tends to form fine particles ofextremely small particle size; hence, a filter element (the type, poresize, and material thereof) suited for collecting the fine particles ofthe impurity must be adequately set. For example, the pore size of thefilter element is preferably about 1 μm or less and more preferably 0.5μm or less. It is also preferable to use a filter aid to increase thefilterability in collecting the fine particles of the impurity. Thefilter aid is not particularly limited, and diatomaceous earth,cellulose fibers, activated charcoal or the like may be used as thefilter aid, for example. Among these, use of activated charcoal ispreferable. The usage of the filter aid is not particularly limited, andboth body feed and precoat filtration can be adequately used.

In contrast, when the impurity is separated as a liquid from the aqueousmedium, a typical liquid-liquid separation, such as centrifugalseparation, may be conducted as necessary to form two layers, i.e., anupper layer and a lower layer, that can be separated from each other,followed by removing the separated liquid as the impurity layer, forexample. When the impurity is soluble in an organic solvent, theimpurity may be removed by distribution in the organic solvent layer.

Furthermore, in a case where separation of the impurities isinsufficient, it is possible to effectively adsorb and remove theimpurities using a known adsorbent such as activated charcoal oractivated clay. Use of activated charcoal as the filtering aid ispreferable since not only the effect of increasing the filterability butalso the effect of removing the impurity by adsorption can be expected.

Next, the second purification process for removing the impurities byadjusting the aqueous medium solution containing the compound (2) orsalt thereof to be neutral to acidic to thereby crystallize a compound(2) in a free form is described.

In the second purification process, the aqueous medium solutioncontaining the compound (2) or salt thereof is adjusted to be neutral toacidic so as to crystallize the compound (2) from the aqueous medium anddissolve the impurity in the aqueous medium. The crystallized compound(2) can be isolated and purified by a typical solid-liquid separationsuch as pressure filtration, vacuum filtration, or centrifugalfiltration. For example, when the impurity is an ionic iron compound,such as iron(III) hydroxide, the solubility thereof under neutral toacidic conditions is higher than that under basic conditions; therefore,it is possible to dissolve the ionic iron compound in the aqueousmedium. In order to adjust the solution to be neutral to acidic, anacidic substance may be added while using the pH of the solution as anindex. The acidic substance added to adjust the solution to be neutralto acidic is not particularly limited and any typical acidic substancemay be used. The upper limit of the pH is usually 9 or less, preferably8 or less, and more preferably 7 or less from the standpoint ofmaintaining the recovery ratio of the compound (2). When the impuritiesare ionic iron compounds such as iron(III) hydroxide, the pH is usually5 or less, preferably 4 or less, and more preferably 3 or less from thestandpoint of removing the impurities. The lower limit of pH is usually1 or more from the standpoint of maintaining the recovery ratio of thecompound (2).

Lastly, the third purification process for removing the impurities byadjusting the solution containing the compound (2) or salt thereof andan aqueous medium to be highly acidic so as to crystallize the salt withthe acid of the compound (2) is described.

In the third purification process, the aqueous medium solutioncontaining the compound (2) or salt thereof is adjusted to be highlyacidic to crystallize the salt with the acid of the compound (2) fromthe aqueous medium and to dissolve the impurities in the aqueous medium.The third purification method is suitably applied to a basic compoundhaving a basic group or an amphoteric compound having both an acidicgroup and a basic group, e.g., optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) represented by saidformula (2a). The crystallized salt with the acid of the compound (2)can be isolated and purified by a typical solid-liquid separation suchas pressure filtration, vacuum filtration, or centrifugal filtration.For example, when the impurities are ionic iron compounds such asiron(III) hydroxide, the impurities have a higher solubility underhighly acidic conditions than the salt with the acid of the compound(2). Thus, the ionic iron compounds can be dissolved in the aqueousmedium. In order to adjust the solution to be acidic, an acidicsubstance may be added while using the equivalent weight relative to thecompound (2) as an index. The acidic substance added to adjust thesolution to be highly acidic is not particularly limited but ispreferably a highly acidic substance having an ionization exponent (pK)of 1 or less. A highly acidic substance commonly used may be used. Theequivalent weight of the acidic substance added cannot be uniformlydefined since it differs depending on the type of acidic substanceadded. However, the equivalent weight is preferably 1 or more times,more preferably 5 or more times, and most preferably 10 or more timesthe compound (2) in terms of molar equivalents. When the pH of thesolution is used as an index, the upper limit of the pH is preferably 1or less and more preferably 0 or less from the standpoint of maintainingthe yield.

The three purification processes described above will now bespecifically described by using the compound (2a) as an example.

A hydrochloride of the compound (3a) and iron(III) chloride aredissolved in water, and the pH of the solution is adjusted to 10 byaddition of sodium hydroxide. The reaction is then allowed to proceedwhile blowing air into the liquid phase to thereby quantitativelyproduce the compound (2a). With the progress of the reaction, the pH ofthe reaction solution increases and reaches 11 at the completion of thereaction. Reddish-brown fine particles are precipitated as a result.

The resulting solution is filtered with a cellulose acetate membranefilter having a pore size of 0.45 μm or with cellulose fibers, activatedcharcoal, or the like as the filtering aid to suitably filter out thereddish-brown fine particles. The resulting filtrate containing the saltwith the base of the compound (2a) thus has a satisfactorily low ironcontent. The iron component can be efficiently removed. This is anactual example of the first purification process.

Meanwhile, the compound (2a) is precipitated as a solid directly fromthe reaction solution by adding concentrated hydrochloric acid to thesolution to control the pH of the solution to 3. Here, the ironcomponents are adequately dissolved in the filtrate. Thus, by filteringand separating the compound (2a), the iron can be efficiently removed.This is the second purification process.

A hydrochloride of the compound (2a) is crystallized as a solid directlyfrom the reaction solution by adding concentrated hydrochloric acid tothe solution to adjust the pH to zero. Here, the iron components areadequately dissolved in the filtrate. Thus, by filtering and separatingthe hydrochloride of the compound (2a), the iron components can beefficiently removed. This is the third purification process.

The three purification processes described above may be used alone or incombination of two or three. The combination of the three purificationprocesses is not particularly limited. Preferably, the firstpurification process is conducted before the second or thirdpurification process. More preferably, the first purification process isconducted before the second purification process. By adjusting the pH asnecessary, the resulting high-purity compound (2) or salt thereof can befreely converted to any form selected from the following three: a saltwith the base of the compound (2), a free form of the compound (2), anda salt with the acid of the compound (2).

The high-purity compound (2) or salt thereof obtained by theabove-described purification processes is used in the subsequentreductive reaction to adequately produce a high-purity compound (1),which is the object of the present invention.

Next, a process of cleaving the sulfur-sulfur bond of the3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative (2) or saltthereof by reductive reaction is described.

Any reductive reaction that can cleave the sulfur-sulfur bond can beemployed without any limitation. For example, chemical reductionprocesses that use, as the reductants, alkali metals such as sodium;acids and metals such as zinc and tin; metal hydride reagents such aslithium aluminum hydride and boron sodium hydride; alkali metal sulfidesuch as potassium sulfide; and phosphine compounds, and electrolyticreduction processes may be employed. In all of these reductionprocesses, it is possible to cleave the sulfur-sulfur bond of thecompound (2) or salt thereof to produce the2-amino-3-mercapto-2-methylpropionic acid derivative (1).

As is previously stated in the description of the Related Art, thecompound (1) is relatively unstable, and the impurities conceivablyderived from the compound (1) are tend to be generated as theby-products. However, according to the reductive reaction of the presentinvention, it is possible to obtain a high purity compound (1)substantially free of the above-described impurities. Moreover, it hasbeen found that the compound (1) is relatively unstable and tends toproduce impurities as the by-products under the basic conditions. Thus,by adjusting the reductive reaction solution to be neutral to acidic,more preferably by maintaining the solution under neutral to acidicconditions through the reductive reaction process, it becomes possibleto increase the stability of the compound (1) to more highly suppressthe generation of the impurities as the by-products. The upper limit ofpH is usually 9 or less, preferably 7 or less, and more preferably 5 orless.

The method for adjusting the reductive reaction solution to be neutralto acidic is not particularly limited. An acidic substance may be addedbefore the initiation of the reductive reaction or during the reactionso that the reaction is conducted in the presence of the acidicsubstance, or after the completion of the reaction. When the neutral toacidic conditions are maintained through the reductive reaction process,it is preferable to allow the acidic substance to coexist before theinitiation of the reductive reaction and to optionally add the acidicsubstance during or after the reaction as necessary. The acidicsubstance to be added is not particularly limited but is preferably ahighly acidic substance. Specific examples thereof include inorganicacids such as hydrohalic acid, e.g., hydrochloric acid, sulfuric acid,sulfurous acid, and nitric acid; sulfonic acids such as methanesulfonicacid, p-toluenesulfonic acid, and o-, m-, or p-nitrobenzenesulfonicacid; and carboxylic acids such as trifluoroacetic acid. Among these,hydrohalic acid is preferred, and hydrochloric acid is particularlypreferred.

Of the reduction methods described above, chemical reduction methods areconvenient since they do not require special facilities as do theelectrolytic reduction processes. In particular, the chemical reductionmethod that use a metal such as zinc or tin with an acid, or a phosphinecompound as the reductant can adequately use the compound (1) unstableunder the basic conditions since reductive reaction can be relativelyeasily proceeded under neutral to acidic conditions according to thesemethods. In particular, a chemical reduction method that uses aphosphine compound as the reductant is preferable since removal of theby-products (impurities) derived from the reductant is easy. Theoperational conditions are described below by using a chemical reductionmethod that uses a phosphine compound as an example.

The phosphine compound usable as the reductant is not particularlylimited but is usually preferably a tertiary phosphine compound.Preferable examples thereof include triarylphosphines such assubstituted or unsubstituted triphenylphosphine and trialkylphosphinessuch as tri-n-butylphosphine and tri-n-octylphosphine.Triphenylphosphine is particularly preferable. Note that these phosphinecompounds are converted to corresponding phosphine oxide compounds inthe course of the reductive reaction.

The amount of the phosphine compound used cannot be uniformly definedsince it differs depending on the type of the compound (2) or saltthereof or phosphine compound, and various conditions such as reactiontemperature. From the standpoint of increasing the yield, the amount ispreferably 1 molar equivalent or more relative to the compound (2).However, when the phosphine compound is used in large excess, theexcessive phosphine compound used in the reductive reaction and theimpurities, such as phosphine oxide compounds produced as theby-products in the reductive reaction, derived from the phosphinecompound are increased. Thus, the load of removing these impurities inthe post reaction treatment is also increased. From the standpoint ofeconomy, the upper limit of the amount of the phosphine compound used ispreferably 2 molar equivalents or less, more preferably 1.5 molarequivalents or less, and yet more preferably 1.3 molar equivalents orless relative to the compound (2).

The reaction solvent used for the reductive reaction using the phosphinecompound is not particularly limited, and a typical solvent may be used.The reaction solvent may be water only, an organic solvent only, or amixture of two or more of the solvents. When two or more solvents aremixed, the solvents may form a homogeneous phase or heterogeneoussystem. In particular, when a mixed solvent of water and an organicsolvent is used as the reaction solvent, an organic solvent that isimmiscible with the aqueous medium and that forms a two-phase system(heterogeneous system) is preferably selected since collection of theproducts and removal of the impurities are easy in the post-reactiontreatment.

Examples of the organic solvent include aliphatic hydrocarbons such ashexane and heptane; aromatic hydrocarbons such as toluene and xylene;halogenated hydrocarbons such as methylene chloride and chloroform;ethers such as tert-butyl methyl ether, tetrahydrofuran, 1,4-dioxane,and dimethoxyethane; esters such as ethyl acetate and isopropyl acetate;ketones such as acetone, methyl ethyl ketone, and methyl isobutylketone; alcohols such as methanol, ethanol, isopropanol, and benzylalcohol; and aprotic polar solvents such as acetonitrile,N,N-dimethylformamide, and dimethyl sulfoxide. Of these, aromatichydrocarbons, halogenated hydrocarbons, and ethers immiscible with theaqueous medium are preferable. In particular, aromatic hydrocarbons arepreferable. Among the aromatic hydrocarbons, toluene is particularlypreferable.

The amount of the reaction solvent used cannot be uniformly definedsince it differs depending on the type of the compound (2) or saltthereof or reaction solvent, and various conditions such as reactiontemperature. Typically, the amount is 1,000 times the weight of thecompound (2) or less, preferably 100 times or less, and most preferably10 times or less from the standpoints of yield and the volumeefficiency.

The temperature of the reductive reaction is not particularly limitedbut should be higher than the temperature (solidification point) atwhich the reaction solution does not solidify. From the standpoint ofincreasing the reaction rate, the higher reaction temperature ispreferable. The upper limit of the reaction temperature is notparticularly limited but should not exceed the boiling point of thereaction solution. As a matter of course, it is preferable to conductthe reaction under reflux conditions.

As is stated above, according to the reductive reaction of the presentinvention, the sulfur-sulfur bond of the compound (2) or salt thereofcan be cleaved and the compound (2) or salt thereof can be substantiallyquantitatively converted to the compound (1) or salt thereof at a highreaction conversion ratio. The expected conversion ratio is at least 99%and preferably at least 99.9%.

The sulfur-sulfur bond of the compound (2) or salt thereof can becleaved by the reductive reaction as described above. Y² and Z² may bethe same as or different from Y¹ and Z¹, respectively. However, when Y²and Z² are different from Y¹ and Z¹, respectively, there is need toconvert Y² to Y¹ and/or Z² to Z¹ to produce the compound (1) or saltthereof. Here, the meaning of the phrase “to convert Y² to Y¹ and/or Z²to Z¹” is, for example, to convert the substituted amino group toanother substituted amino group by derivatization of the substituent, toconvert the substituted amino group to an unsubstituted amino group byremoval of the substituent, or to convert the unsubstituted amino groupto a substituted amino group by introduction of a substituent. When Y²and Z² together form a divalent group, this phrase means that, forexample, the divalent group is converted to another divalent group byderivatization or to a monovalent group indicated by Y¹ and Z¹ above.When conversion of Y² to Y¹ and/or Z² to Z¹ is performed, the conversionmay take place after the completion of the reductive reaction of thecompound (2) or salt thereof. Alternatively, Y² of the compound (2) maybe converted to Y¹, and/or Z² to Z¹ before the reductive reaction. Yetalternatively, Y² may be converted to Y¹ and/or Z² to Z¹ simultaneouslywith the reductive reaction.

Conversion of Y² to Y¹ and/or Z² to Z¹ can be conducted by the samemethod as that employed for converting Y³ to Y² and/or Z³ to Z².

The reductive reaction of the present invention can be equally appliedto the compound (2), which is the impurity contained in the targetcompound (1) of the present invention. For example, in the case in whichthe reductant is deactivated by unexpected oxygen contamination duringthe reductive reaction and the reaction is thereby stopped beforesufficiently achieving the expected conversion ratio, the reductivereaction of the present invention may be performed once again tocomplete the reaction so that the expected conversion ratio is attained.Another preferable example of the method for purifying the compound (1)containing the compound (2) as the impurity is a method for convertingthe compound (2), which is the impurity, to the compound (1) by thereductive reaction of the present invention. The content of the compound(2) as the impurity is not particularly limited. Even when the contentof the compound (2) is 1% or more or 0.1% or more, the compound (2)content can be decreased by the reductive reaction of the presentinvention.

As is stated above, with the reductive reaction of the presentinvention, it is also possible to convert the compound (2) or saltthereof contained as the impurity in the compound (1) or salt thereof tothe compound (1) or salt thereof at a high reaction conversion ratio bycleaving the sulfur-sulfur bond of the compound (2) or salt thereof.

Note that in the reductive reaction, as a precautionary measure, it ispreferable to purge the interior of the reactor with inert gas such asnitrogen or argon to reduce the oxygen concentration in the reactionsystem from the standpoint of suppressing the inactivation of and theside reaction caused by the oxidation of the compound (1), which is thereduction product. The oxygen concentration in the reactor is usually0.5% or less, preferably 0.2% or less, and more preferably 0.1% or less.

Next, the purification process for removing the impurities contained inthe compound (1) obtained by the reductive reaction is explained. Forexample, in the case of producing the compound (1) by reducing thecompound (2) by using a phosphine compound as the reductant, thephosphine compound, which is the reductant, and phosphine oxidecompounds derived from the reductant are generated as impurities. Inaddition, impurities possibly derived from the compounds (1) and (2) arealso produced as-the by-products. Such impurities are frequentlyfat-soluble and have a high solubility in various organic solvents.Among these, the phosphine compound and the components (impurities suchas phosphine oxide compounds) derived from the reductant are generallyfat-soluble and exhibit high solubility in various organic solvents anda high distribution ratio during extraction. In contrast, the compound(1) has a high solubility in water regardless of the pH and is difficultto extract with various organic solvents.

Thus, the aqueous medium solution of the compound (1) containing thefat-soluble impurities may be washed with an organic solvent immisciblewith the aqueous medium so as to efficiently remove the fat-solubleimpurities to the organic solvent layer. Thus, the aqueous mediumsolution containing the compound (1) or salt thereof can be purified.

The organic solvent used for the washing is not particularly limited andmay be any common organic solvent. For example, reaction solvents usablein the reductive reaction are preferable as the organic solvent.Particularly preferable examples of the reaction solvent are organicsolvents immiscible with the aqueous solvent and include aliphatichydrocarbons such as hexane and heptane; aromatic hydrocarbons such astoluene and xylene; halogenated hydrocarbons such as methylene chlorideand chloroform; ethers such as tert-butyl methyl ether and 1,4-dioxane;esters such as ethyl acetate and isopropyl acetate; and ketones such asmethyl isobutyl ketone. Of these, aromatic hydrocarbons, halogenatedhydrocarbons, and ethers are particularly preferable, and aromatichydrocarbons are still more preferable. Among the aromatic hydrocarbons,toluene is particularly preferable.

The method of washing with the organic solvent is not particularlylimited. In general, an organic solvent for washing is brought intocontact with the aqueous medium solution containing the compound (1) toextract the fat-soluble impurities into the organic solvent phase,followed by separating and removing the resulting organic solvent layer.As a matter of course, the reaction solvent used in the reductivereaction may be directly used as the organic solvent for washing.Alternatively, the organic solvent may be directly added after thecompletion of the reductive reaction or added after the reaction solventis distilled away (solvent substitution) as necessary.

The pH of the aqueous medium solution containing the compound (1) whenbrought into contact with the organic solvent is not particularlylimited. It is preferable to adequately adjust the pH of the aqueousmedium solution to increase the yield of the compound (1) and theimpurity removal ratio. The adequate pH cannot be uniformly determinedsince it differs depending on the type of the compound (1). In the caseof an amphoteric compound having both an acidic group and a basic group,it is preferable to adjust the pH to out of the range of 4 to 5, morepreferably 3 or less.

An example of the amphoteric compound is an amino acid such as opticallyactive 2-amino-3-mercapto-2-methylpropionic acid represented by formula(1a):

(which is a compound represented by said general formula (1) with Y¹representing an unsubstituted hydroxyl group and Z¹ representing anunsubstituted amino group, hereinafter this compound is also referred toas “compound (1a)”).

When the compound (1) is an acidic compound, the pH is preferablyadjusted to 6 or higher, i.e., neutral to basic. An example of theacidic compound is a hydantoin derivative such as optically active5-mercaptomethyl-5-methylhydantoin represented by formula (1b):

(which is the compound represented by said general formula (1) with Y¹and Z¹ together forming ureylene group, hereinafter this compound isalso referred to as the “compound (1b)”).

As a result of the purification described above, an aqueous mediumsolution containing the compound (1) or salt thereof and from whichvarious impurities are removed can be obtained. Accordingly, when theaqueous medium solution of the compound (1) produced by the presentinvention is used, solid of a high-quality salt with the acid of thecompound (1) can be obtained by solidification and isolation accordingto the isolation method set forth in WO01/72702, which has not beenpossible by the conventional production process.

Alternatively, the compound (1) or salt thereof can be crystallized inthe presence of an organic solvent. In particular, the compound (1) orsalt thereof can be adequately crystallized by concentrating the waterin the presence of an organic solvent to replace water with the organicsolvent.

The compound (1) or salt thereof is not particularly limited. Examplesthereof include the compound (1), a salt with an acid of the compound(1), and a salt with a base of the compound (1). A salt with an acid isparticularly preferred.

The acid in the salt with the acid of the compound (1) is notparticularly limited but is preferably highly acidic. In particular,inorganic acids such as hydrohalic acid, e.g., hydrochloric acid,sulfuric acid, sulfurous acid, and nitric acid; sulfonic acids such asmethanesulfonic acid, p-toluenesulfonic acid, and o-, m-, orp-nitrobenzenesulfonic acid; and carboxylic acids such astrifluoroacetic acid are preferable. Of these, hydrohalic acid ispreferred, and hydrochloric acid is particularly preferred.

Examples of the base in the salt with the base include amines such asammonia, triethylamine, aniline, and pyridine.

In conducting the crystallization method, the aqueous medium solutioncontaining the compound (1) or salt thereof may be preliminarilyconcentrated prior to the addition of the organic solvent. The type ofthe organic solvent for substitution is not particularly limited. Anorganic solvent azeotropic with water is preferred, and an organicsolvent immiscible with water is more preferred.

The organic solvent is not particularly limited. Examples of the organicsolvent include aliphatic hydrocarbons such as hexane and heptane;aromatic hydrocarbons such as toluene and xylene; halogenatedhydrocarbons such as methylene chloride and chloroform; ethers such astert-butyl methyl ether, 1,4-dioxane, and dimethoxyethane; and esterssuch as ethyl acetate and isopropyl acetate. Of these, aromatichydrocarbons, esters, and ethers are particularly preferable. Aromatichydrocarbons are yet more preferable since they have low dissolubilityof the aqueous medium and the compound (1) or salt thereof and they areeasy to recover and recycle. Among the aromatic hydrocarbons, toluene isparticularly preferable.

In the substitution by the organic solvent, the distillation and feedingof the organic solvent may be conducted simultaneously (continuousmethod) or alternately over a plurality of times (batch method). Theamount of the organic solvent for substitution is not uniformly definedsince it differs depending on the type of the organic solvent, thedegree of vacuum during the concentration, and the inner temperature.For example, where toluene is concerned, the amount is usually up to 100times, preferably 50 times, and most preferably 10 times the totalweight of the aqueous solution.

The concentration of the compound (1) or salt thereof in the course ofcrystallizing the compound (1) or salt thereof by concentrating water inthe presence of the organic solvent to conduct organic solventsubstitution is preferably 0.1 wt % or more and more preferably 1 wt %or more. The upper limit is usually 70 wt % or less, preferably 50 wt %or less, and most preferably 30 wt % or less.

The final amount of water remaining after the removal of water tooutside the system by the above-described operation is preferably 100 wt% or less relative to the compound (1) or salt thereof. From thestandpoints of properties of the resulting crystals, filterability,crystal recovery ratio, and fluidity of the slurry, it is preferable todecrease water to 40 wt % or less.

The rate of evaporation for concentration depends on the shape andperformance of the equipment, so it is not particularly defined. At ahigh evaporation rate, severe bubbling occurs, the resulting slurryexhibits extremely poor fluidity, and the crystals of the compound (1)tend to form aggregates/agglomerates. Thus, the evaporation rate perunit evaporation area and per unit time is preferably controlled to1,000 L/h·m² or lower, more preferably 600 L/h·m² or lower, yet morepreferably 300 L/h·m² or lower, and most preferably 100 L/h·m² or lower.

The degree of vacuum for concentration after the addition of the organicsolvent differs depending on the type of the organic solvent but isusually 500 mmHg or less and preferably 200 mmHg or less. The lowerlimit is not particularly limited but is usually 0.1 mmHg or more.

The temperature for the concentration depends on the degree of vacuumand performance of the equipment but usually 0° C. to 150° C.,preferably 10° C. to 100° C., and more preferably 30° C. to 70° C. sothat the handling is easy and high-quality crystals can be obtained.

As is stated above, according to the present invention, the compoundrepresented by said general formula (2) or salt thereof and the compoundrepresented by said general formula (1) or salt thereof can be easilyand efficiently produced from the compound represented by said generalformula (3) or salt thereof on an industrial production scale whilehighly suppressing the contamination by impurities.

The combination of the compounds of the present invention is notparticularly limited. The compounds (1), (2), and (3) can be freelycombined to conduct the reaction. The most preferable combination of thecompounds is, for example, to form a sulfur-sulfur bond betweenmolecules of optically active 5-mercaptomethyl-5-methylhydantoinrepresented by said formula (3b) or salt thereof so as to produce thecompound (2′b) or salt thereof, to hydrolyze the compound (2′b) or saltthereof to cleave the hydantoin ring to thereby produce the compound(2a) or salt thereof, and to then reduce the compound (2a) or saltthereof to cleave the sulfur-sulfur bond so as to produce the compound(1a) or salt thereof.

According to the present invention, it is possible to purify alow-quality compound represented by said general formula (3) or saltthereof or a low-quality compound represented by said general formula(1), containing the compound represented by said general formula (2) orsalt as an impurity. The content of the compound (2), which is theimpurity contained in the compound (3), is not particularly limited.Even when the content is 1% or more or 0.1% or more, the content of thecompound (2) can still be decreased by the above-described purificationprocess.

For example, when the low-quality compound (3) or salt thereof containswater-soluble impurities, such as inorganic substances, that aredifficult to remove from the compound (1), a high-quality compound (1)can be obtained by the process of the present invention, i.e., theprocess including converting the low-quality compound (3) to thecompound (2) by oxidation, removing the water-soluble impurities bypurification, and quantitatively reducing the resulting compound (2) toobtain the high-quality compound (1).

The content of the compound (2) as the impurity in the compound (3) isnot particularly limited. Even at a compound (2) content of 1% or moreor 0.1% or more, the compound (2) content can be reduced by theabove-described purification process. In contrast, in the case of thelow-quality compound (1) that does not contain impurities difficult toremove from the compound (1) other than the compound (2), the compound(2) contained as the impurity may be quantitatively reduced by theprocess of the present invention to produce the high-quality compound(1).

As is stated above, according to the present invention, a low-qualitycompound (1) or compound (3) can be easily and efficiently purified onan industrial production scale in all instances.

Next, the method for producing the optically active3-mercapto-2-methylpropionic acid derivative (3) or salt thereof used inthe production process of the present invention is described. The methodfor producing the compound (3) or salt thereof is not particularlylimited, and various methods may be employed including those discussedas the examples of the related art. In view of the object of the presentinvention, a method suitable for industrial production is preferablyused among these methods. An example of the preferable method is onedescribed in WO03/106689, in which racemic2-carbamoylamino-3-mercapto-2-methylpropionic acid or salt thereof isreacted with hydantoinase to selectively cyclize the D isomers and tothereby produce a D-5-mercaptomethyl-5-methylhydantoin derivative orsalt thereof and L-2-carbamoylamino-3-mercapto-2-methylpropionic acid orsalt thereof.

According to the above-described method, the optically active3-mercapto-2-methylpropionic acid derivative (3) or salt thereof can beeasily and efficiently produced on an industrial production scale. Thus,according to the present invention, the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative (2) or saltthereof and the optically active 2-amino-3-mercapto-2-methylpropionicacid derivative (1) or salt thereof can be easily and efficientlyproduced from the optically active 3-mercapto-2-methylpropionic acidderivative (3) or salt thereof on an industrial production scale whilehighly suppressing the contamination by the impurities.

EXAMPLES

The present invention will now be described in further detail by way ofexamples. It is to be understood that the present invention is notlimited to these examples.

Various analyses in Examples were conducted under the followingconditions: HPLC-1 [Column: Cosmosil 5C18-ARII (produced by NacalaiTesque Inc.), mobile phase: potassium dihydrogen phosphate.phosphoricacid aqueous solution (pH 2.0)/acetonitrile=97/3, flow rate: 1.0 ml/min,detection wavelength: 210 nm, column temperature: 40° C.]; HPLC-2[Column: Cosmosil 5C18-AR (produced by Nacalai Tesque Inc.), mobilephase: potassium dihydrogen phosphate.phosphoric acid.sodiumheptanesulfonate aqueous solution (pH 2.5) /acetonitrile=95/5, flowrate: 1.0 ml/min, detection wavelength: 210 nm, column temperature: 40°C.]; HPLC-3 [Column: Cosmosil 5C8-MS (produced by Nacalai Tesque Inc.)150 mm×4.6 mm I.D., mobile phase: potassium dihydrogen phosphate.sodiumhydroxide aqueous solution (pH 6.8)/acetonitrile=1/1, flow rate: 1.0ml/min, detection wavelength: 215 nm, column temperature: 40° C.];HPLC-4 [Column: CAPCELL PAK SCX (produced by Shiseido Co., Ltd.) 250mm×4.6 mm I.D., mobile phase: potassium dihydrogen phosphate.phosphoricacid aqueous solution (pH 2.0)/acetonitrile=95/5, flow rate: 0.3 ml/min,detection wavelength: 210 nm, column temperature: 40° C.]; Chiral HPLC-1[Column: CHIRALPAK AS (produced by Daicel), mobilephase:hexane/isopropanol/trichloroacetic acid=9/1/0.01, flow rate: 0.5 ml/min,detection wavelength: 210 nm, and column temperature: 30° C.]; ChiralHPLC-2 [Column: CHIRALPAKAS (produced by Daicel), mobile phase:hexane/isopropanol=9/1, flow rate: 1.0 ml/min, detection wavelength: 210nm, column temperature: 30° C.]; Specific optical rotation [Lightsource: sodium lamp, cell length: 10 cm]; Iron content (atomicabsorption spectrometry) [Atomization method: flame method(air-acetylene), detection wavelength: 248.3 nm (Fe)]; Sodium content(ion chromatography) [Column: TSKgel IC-Cation (produced by TOSOH),mobile phase: 2 mM nitric acid aqueous solution, flow rate: 1.0 ml/min,detector: electric conductivity, column temperature: 40° C.]; IR[Equipment: single beam FT-IR, sample: KBr pellets].

The conversion ratio and the residual ratio in each Example arecalculated by the following equations:conversion ratio=area of product/(area of starting material+area ofproduct)×100 (%)residual ratio=area of starting material/(area of starting material+areaof product)×100 (%)

Reference Example 1 Preparation of Racemic5-tert-butylthiomethyl-5-methylhydantoin

Under a nitrogen atmosphere, 9.6 g of 5 wt % aqueous sodium hydroxidesolution and 1.13 mL of tert-butyl mercaptan were combined at 0° C. andstirred for 10 minutes. To the resulting mixture, 0.79 mL ofchloroacetone was added, and the resulting mixture was warmed to roomtemperature and allowed to react for two hours. The reaction solutionwas buff yellow and separated into two phases.

To the reaction solution, 588 mg of NaCN, 2.77 g of (NH₄) HCO₃, and 3.1mL of 30 wt % aqueous ammonia were added to prepare a homogeneoussolution, followed by heating to 55° C. to 60° C. After the solution washeated for 6 hours under stirring, the reaction solution was cooled to0° C. and combined with concentrated hydrochloric acid to adjust the pHto 7.0 to 7.6. White crystals thereby generated were filtered out, and1.84 g of racemic 5-tert-butylthiomethyl-5-methylhydantoin was obtainedas a result.

Reference Example 2 Preparation of racemic2-carbamoylamino-3-tert-butylthio-2-methylpropionic acid

In 75 g of 10% aqueous solution of sodium hydroxide, 4.77 g of racemic5-tert-butylthiomethyl-5-methylhydantoin was dissolved and the solutionwas refluxed for 72 hours. There action solution was cooled to roomtemperature and then a portion of the reaction solution was sampled. Ageneration of racemic 2-amino-3-tert-butylthio-2-methylpropionic acidwas confirmed from the sample by HPLC. The reaction solution wasadjusted to a pH of 8 by concentrated hydrochloric acid and heated to70° C. To the resulting reaction solution, a solution of 2.07 g ofpotassium cyanate in 10 mL of distilled water was added dropwise over 20minutes. After the completion of the addition, stirring was conductedfor 5 hours, and a portion of the reaction solution was sampled andsubjected to HPLC analysis (HPLC-1). Unreacted racemic2-amino-3-tert-butylthio-2-methylpropionic acid was detected as aresult. Thus, a solution of 4.14 g of potassium cyanate in 20 mL ofdistilled water was further added to the reaction solution dropwise over20 minutes. Upon completion of the addition, stirring was conducted for1 hour and the reaction solution was cooled to room temperature. The pHwas adjusted to 2 with concentrated hydrochloric acid, and the soliddeposits were filtered out. The resulting solid was washed with waterand dried. As a result, 3.38 g of racemic2-carbamoylamino-3-tert-butylthio-2-methylpropionic acid was obtained.

Reference Example 3 Preparation ofL-2-carbamoylamino-3-tert-butylthio-2-methylpropionic acid andD-5-tert-butylthiomethyl-5-methylhydantoin

In accordance with the culture method and preparation method ofimmobilized enzyme set forth in WO96/20275, the strain Bacillus sp.KNK245 (name of the depository institution to which the strain wasdeposited: National Institute of Bioscience and Human-Technology, Agencyof Industrial Science and Technology, METI, Address: 1-1-3 Tsukuba-shi,Ibaraki Japan (zip code: 305), Date of accession: Nov. 2, 1994,Accession number under which the strain was deposited with thedepository institution: FERM BP-4863) was cultured and the culturedcells were collected. To an enzymatic solution obtained by ultrasonicdisintegration of the cells, an anionic exchange resin serving as asupport for immobilization, namely, Duolite A-568, was added to adsorbthe enzyme, followed by cross-linking treatment with glutaric aldehyde.Immobilized hydantoinase was obtained as a result.

Next, to 15 mg of racemic2-carbamoylamino-3-tert-butylthio-2-methylpropionic acid prepared inReference Example 2, 1.5 ml of 0.1 M potassium phosphate buffer solution(pH=7.0) and 0.003 ml of 0.5 M aqueous manganese sulfate solution wereadded, the pH of the resulting solution was adjusted to 6.5 with 10Naqueous sodium hydroxide solution. To this solution, 200 mg (wet weight)of the immobilized hydantoinase obtained above was added, and theresulting mixture was allowed to react at 40° C. for 48 hours understirring. During the reaction, the pH was maintained at about 6.5 with6N hydrochloric acid. The reaction solution was subjected to HPLCanalysis (HPLC-1), and it was found that the residual ratio of2-carbamoylamino-3-tert-butylthio-2-methylpropionic acid was 41%. Theoptical purity of 2-carbamoylamino-3-tert-butylthio-2-methylpropionicacid in the reaction solution was determined by HPLC analysis (chiralHPLC-1), and it was found thatL-2-carbamoylamino-3-tert-butylthio-2-methylpropionic acid was producedin an optical purity of 96.7% ee. Meanwhile, the compound generated andprecipitated during the enzymatic reaction described above was extractedwith ethyl acetate and subjected to HPLC analysis (chiral HPLC-2). Itwas found that the compound wasD-5-tert-butylthiomethyl-5-methylhydantoin.

L-2-Carbamoylamino-3-tert-butylthio-2-methylpropionic acid: ¹H NMR (300MHz, CD₃OD) δ: 3.22 (d,1H), 3.16 (d,1H), 1.52 (s,3H), 1.29 (s,9H)

D-5-tert-Butylthiomethyl-5-methylhydantoin: ¹H NMR (300 MHz, CDC1₃ with3 drops of CD₃OD) δ: 2.90 (d,1H), 2.80 (d,1H), 1.49 (s,3H), 1.30 (s,9H).

Reference Example 4 Preparation ofL-2-carbamoylamino-3-tert-butylthio-2-methylpropionic acid usingbacteria of the genus bacillus

Dry preserved cells of the strain Bacillus sp. KNK245 (FERM BP-4863)were inoculated in 100 ml of a liquid culture medium (10 g/lpolypeptone, 10 g/l meat extract, 5 g/l yeast extract, pH: 7.5)sterilized at 120° C. for 15 minutes in a 500 mL Sakaguchi flask, andshake culture was conducted at 45° C. for 15 hours. Two milliliters ofthe cultured solution was inoculated in a culture medium in which 1 g/lof uracil and 20 mg/l of manganese chloride were added to theabove-described culture components, and shake culture was performed at45° C. for 24 hours. Cells obtained from 15 ml of the cultured solutionby centrifugal separation were suspended in 1.5 ml of 0.1 M potassiumphosphate buffer solution (pH: 7.0), and combined with 150 mg of racemic2-carbamoylamino-3-tert-butylthio-2-methylpropionic acid and 0.003 ml of0.5 M aqueous manganese sulfate solution. Subsequently, the pH wasadjusted to 6.5 with 10N aqueous sodium hydroxide solution. Whilemaintaining the pH to about 6.5 with 6N hydrochloric acid, reaction wasconducted at 40° C. for 19 hours under stirring. The results of the HPLCanalysis of the reaction solution showed that the residual ratio of2-carbamoylamino-3-tert-butylthio-2-methylpropionic acid was 44%. Theoptical purity of 2-carbamoylamino-3-tert-butylthio-2-methylpropionicacid in the reaction solution was also determined by HPLC analysis(chiral HPLC-1). It was confirmed thatL-2-carbamoylamino-3-tert-butylthio-2-methylpropionic acid

Reference Example 5 Preparation ofD-5-tert-butylthiomethyl-5-methylhydantoin

In order to remove an impurity, i.e.,2-amino-3-tert-butylthio-2-methylpropionic acid, contained in 50 g of amixture of the enzyme and D-5-tert-butylthiomethyl-5-methylhydantoinobtained by the method of Reference Example 3, 400 g of water was added.After the mixture was stirred, the insoluble components were filteredout and further washed with 200 g of water. 5 wt % aqueous sodiumhydroxide solution (120 g) was added thereto, and the resulting mixturewas stirred. The enzyme was filtered out as the insoluble component, andthe pH of the filtrate was adjusted to 9 with concentrated hydrochloricacid. The crystals precipitated were filtered out, washed with water,and dried under a reduced pressure to obtain 19.7 g of a crude productas crystals. The crystals were analyzed by HPLC and were found to have apurity of 87.5 wt %. The optical purity thereof determined by HPLCanalysis (chiral HPLC-2) was 97.6% ee.

Reference Example 6 Preparation of D-5-mercaptomethyl-5-methylhydantoin

D-5-tert-Butylthiomethyl-5-methylhydantoin (4.38 g) obtained inReference Example 4 was dissolved in 100 g of concentrated hydrochloricacid, and the solution was allowed to react at 80° C. for 18.5 hours.After the solution was cooled to room temperature, the solution wasconcentrated to about half and combined with 30.5 g of 30 wt % aqueoussodium hydroxide solution to adjust the pH to 10. After extraction withethyl acetate (100 mL×3), the whole organic phase was concentrated to10% of the total amount, and 30 mL of toluene was added the residue todeposit crystals. The crystals were filtered out, and 2.65 g of targetD-5-mercaptomethyl-5-methylhydantoin was obtained as a result. Theoptical purity of this substance was measured by HPLC (chiral HPLC-2).No L isomer was detected as a result.

¹H NMR (400 MHz, MeOH-d4) δ: 1.32 (s, 3H), 2.60 (d, 1.6 Hz,1H), 2.72 (d,1.6 Hz, 1H)

Reference Example 7 Preparation ofD-2-amino-3-tert-butylthio-2-methylpropionic acid

To 80 g of a mixture of D-5-tert-butylthiomethyl-5-methylhydantoin andthe enzyme obtained as in Reference Example 5, 150 mL of 10 wt % lithiumhydroxide aqueous solution was added to dissolve the mixture. Afterfiltering out the enzyme, the quantity ofD-5-tert-butylthiomethyl-5-methylhydantoin in the mother liquid wasdetermined by HPLC. As a result, 44.2 g ofD-5-tert-butylthiomethyl-5-methylhydantoin was contained. To thissolution, 54 g of lithium hydroxide and 51 g of distilled water wereadded, and the resulting mixture was refluxed under heating for 38hours. After cooling to room temperature, solids generated werefiltered. While maintaining the inner temperature of a mother liquid atabout 20° C., 110 g of concentrated hydrochloric acid was added toadjust the pH to 6.7, followed by cooling to an inner temperature of 2°C. and stirring for two hours. Subsequently, the solids generated werefiltered, and vacuum-dried at 40° C. for 24 hours. As a result, 34.9 gof dry crystals were obtained. The crystals were analyzed by HPLC(HPLC-1) and confirmed to be the title compound (purity: 96.7 wt %).

Reference Example 8 Preparation ofL-2-amino-3-tert-butylthio-2-methylpropionic acid

L-2-Carbamoylamino-3-tert-butylthio-2-methylpropionic acid (82.4 g) wasdissolved in 630 g of 18% lithium hydroxide aqueous solution, and theresulting solution was refluxed for 41 hours under nitrogen. Thesolution was cooled to room temperature and filtered to remove theinsoluble components. To the resulting solution, 180.1 g of concentratedhydrochloric acid was added to adjust the pH to 6. The solution wasstirred for about 1 hour, then cooled to 4° C. to 5° C., and againstirred for 1 hour. The generated crystals were filtered out, washedwith water, and dried under a reduced pressure. As a result, 53.9 g ofwhite solid was obtained.

¹H NMR (300 MHz, D₂O) δ: 3.18 (d, 1H), 2.91 (d, 1H), 1.60 (s, 3H), 1.35(s, 9H)

Reference Example 9 Preparation ofL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride

To 38.4 g of L-2-amino-3-tert-butylthio-2-methylpropionic acid obtainedby the method of Reference Example 8, 345.3 g of concentratedhydrochloric acid was added, and the resulting mixture was refluxed for24 hours to obtain an aqueous solution ofL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride.

The aqueous solution was concentrated to 67.5 g (degree of vacuum: 30 to60 mmHg, temperature: 45° C.) and combined with 206 g of toluene. Theresulting mixture was subjected to vacuum concentration (degree ofvacuum: 40 to 60 mmHg, temperature: 40° C., distillation rate: 107L/h·m²) so that the total weight was 109 g. Toluene (206 g) was addedthereto to conduct concentration and the same operation was performed atotal of six times to obtain 104 g of a toluene slurry ofL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride. The slurrycontained 30 wt % of water relative toL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride. Crystals werefiltered, washed with toluene, and vacuum-dried to obtain 32.2 g ofwhite solid of L-2-amino-3-mercapto-2-methylpropionic acidhydrochloride.

¹H NMR (300 MHz, D₂O) δ: 3.18 (d, 1H), 2.89 (d, 1H), 1.60 (s, 3H)specific rotation: [α]^(D) ₂₀=+8.770° (cl.15, H₂O)

The solid was confirmed to be the L stereoisomer since the sign of thespecific rotation was coincident with that in the literature(Tetrahedron, 1993, 49, 2131-2138, WO98/38177).

Example 1 Preparation of(5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin)

In 1,000 g of concentrated hydrochloric acid, 50 g ofD-5-tert-butylthiomethyl-5-methylhydantoin obtained as in ReferenceExample 5 was dissolved, and the solution was allowed to react at 80° C.for 18 hours to obtain an aqueous solution ofD-5-mercaptomethyl-5-methylhydantoin. The residual ratio ofD-5-tert-butylthiomethyl-5-methylhydantoin was 1% (HPLC-2).

The resulting reaction solution was concentrated to 208 g and combinedwith 133 g of 30 wt % aqueous sodium hydroxide solution at roomtemperature to adjust the pH to 9.0. To this solution, 13 mg(corresponding to 4 mg of iron) of iron trichloride was added, and thereaction was conducted under vigorous stirring with air bubbling at roomtemperature for 4 days to obtain 270 g of an aqueous solution containing29.3 g of (5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin).

The iron content in the resulting aqueous solution was 16 ppm(corresponding to 4 mg of iron), andD-5-mercaptomethyl-5-methylhydantoin was not detected (less than 0.1% byHPLC-2).

Example 2 Removing Iron From(5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin) (ActivatedCharcoal Treatment)

To 231 g (iron content: 4 mg) of the aqueous solution obtained inExample 1, 30 wt % aqueous sodium hydroxide solution was added to adjustthe pH to 11.0. Reddish-brown crystals were precipitated as a result.The solution was filtered through 1.5 g of activate charcoal (producedby Takeda Pharmaceutical Company Limited: activated charcoal ShirasagiA) preliminarily washed with 0.01 M aqueous sodium hydroxide solution,and then further washed with 10 ml of 0.01 M aqueous sodium hydroxidesolution to obtain 238 g of clear liquid. The iron content in the clearliquid was 0.02 ppm (corresponding to 0.006 mg of iron).

To 237 g of the clear liquid, 20 g of concentrated hydrochloric acid wasadded at room temperature to neutralize, thereby adjusting the pH to1.8. Crystals precipitated were filtered, washed with water, andvacuum-dried to obtain 28.5 g of(5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin) as a whitesolid. The iron content in the crystals was 0.01 ppm (corresponding to0.003 mg of iron).

¹H NMR (300 MHz, NaOD/D₂O) δ: 3.15 (d, 1H), 3.05 (d, 1H), 1.35 (s, 3H)

¹³C NMR (300 MHz, NaOD/D₂O) δ: 196.9, 176.2, 68.3, 50.2, 25.6 IR (cm⁻¹,KBr) 1770.5, 1701.1, 1409.9, 1305.7, 1024.1, 773.4, 763.8, 648.0, 578.6,426.2

[α]^(D) ₂₀=+161.23° (c0.53, 1N NaOH)

Example 3 Removing Iron From(5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin) (Filtration)

To 23.1 g of the aqueous solution (containing 0.4 mg of iron) obtainedin Example 1, 30 wt % aqueous sodium hydroxide solution was added toadjust the pH to 11.0. Reddish-brown crystals were precipitated as aresult. The solution was filtered through a membrane filter (pore size:0.45 μm, made of cellulose acetate) to obtain 23.2 g of clear liquid.The iron content in the clear liquid was 0.09 ppm (corresponding to0.002 mg of iron).

Example 4 Crystallization of(5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin)

To 12 g of the aqueous solution (containing 0.2 mg of iron) obtained inExample 1, 10 g of concentrated hydrochloric acid was added at roomtemperature to neutralize, thereby adjusting the pH to 1.8. Theprecipitated crystals were filtered, washed with water, and vacuum-driedto obtain 14.6 g of(5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin) as a whitesolid. The iron content in the resulting crystals was 98 ppm(corresponding to 0.1 mg).

Example 5 Preparation of(2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic acid)

In a 500 mL pressure reactor resistant to hydrochloric acid, 25 g of(5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin) obtained inExample 2 and 160 g of concentrated hydrochloric acid were enclosed andallowed to react for 36 hours in an oil bath at 120° C. HPLC analysis(HPLC-2) was conducted but(5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin) was notdetected in the resulting reaction solution (less than 0.1%).

The resulting reaction solution (pH thereof was less than 0) was cooledto 3° C., and the precipitated crystals were filtered, washed with 25 mLof cold concentrated hydrochloric acid, and vacuum-dried to obtain 23.8g of (2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic acid)hydrochloride as a white solid.

¹H NMR (300 MHz, NaOD/D₂O) δ: 3.31 (d, 1H), 2.92 (d, 1H), 1.30 (s, 3H)

¹H NMR (300 MHz, D₂O) δ: 3.59 (d, 1H), 3.20 (d, 1H), 1.63 (s, 3H)

¹³C NMR (300 MHz, NaOD/D₂O) δ: 185.1, 61.7, 54.0, 28.8

¹³C NMR (300 MHz, D₂O) δ: 175.7, 63.2, 47.1, 24.9

[α]^(D) ₂₀=+40.75° (c0.53, 1N NaOH)

[α]^(D) ₂₀=−148.35° (c1.02, 1N HCl)

Example 6 Preparation of D-2-amino-3-mercapto-2-methylpropionic Acid byReduction of (2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic Acid)

To 5.0 g of (2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic acid)obtained in Example 5 and 6.4 g of triphenylphosphine, 50 g of tolueneand 6.4 g of concentrated hydrochloric acid were sequentially added atroom temperature under nitrogen stream. The mixture was then heated to80° C. Subsequently, a total of 7.75 g of water was added in two dividedportions to completely dissolve crystals of(2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic acid). The reactionwas further conducted for 24 hours to obtain 67 g of reaction solutioncontaining 5.9 g of D-2-amino-3-mercapto-2-methylpropionic acidhydrochloride. The residual ratio of(2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic acid) in the resultingreaction solution was 0.3% (HPLC-3).

An aqueous solution (pH: 0) obtained by separating the toluene layerfrom this reaction solution was washed with 50 g of toluene four times.Triphenylphosphine and triphenylphosphine oxides were no longer detectedas a result (less than 0.01% by HPLC-3) in the aqueous solution. Thus,aqueous D-2-amino-3-mercapto-2-methylpropionic acid hydrochloridesolution with an optical purity of 99.6 area % was obtained.

The aqueous solution was concentrated (degree of vacuum: 30 to 60 mmHg,temperature: 45° C.) to 13 g and combined with 40 g of toluene. Vacuumconcentration was then conducted (degree of vacuum: 40 to 60 mmHg,temperature: 40° C., and distillation rate: 107 L/h·m²) to bring thetotal weight to 15 g. Toluene (40 g) was further added thereto andconcentration was conducted. The similar operation was carried out atotal of five times. To the resulting solution, toluene was added toobtain 184 g of toluene slurry of D-2-amino-3-mercapto-2-methylpropionicacid hydrochloride. Crystals were′filtered, washed with toluene, andvacuum-dried to obtain 5.9 g of D-2-amino-3-mercapto-2-methylpropionicacid hydrochloride as a white solid. As a result of HPLC analysis(HPLC-4), the purity of the resulting crystals ofD-2-amino-3-mercapto-2-methylpropionic acid hydrochloride was 99.6 area%, and the content of (2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionicacid) was 0.3 area %.

¹H NMR (300 MHz, D₂O) δ: 3.18 (d, 1H), 2.89 (d, 1H), 1.60 (s, 3H)

[α]^(D) ₂₀=−8.76° (c1.01, H₂O)

Example 7 Preparation of D-5-mercaptomethyl-5-methylhydantoin byReduction of (5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin)

To 5.9 g of (5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin)obtained in Example 4 and 6.4 g of triphenylphosphine, 50 g of toluene,15.5 g of water, and 6.4 g of concentrated hydrochloric acid weresequentially added at room temperature under nitrogen stream, and themixture was allowed to react for 24 hours at 80° C. The resultingreaction solution was analyzed by HPLC (HPLC-3). The residual ratio of(5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin) was 0.9%.

To this reaction solution, 30 wt % aqueous sodium hydroxide solution wasadded to adjust the pH to 9.0. An aqueous solution obtained byseparating the toluene layer was washed with 50 g of toluene four timesto obtain 22 g of an aqueous solution.

As a result of HPLC analysis, the aqueous solution contained 5.8 g ofD-5-mercaptomethyl-5-methylhydantoin and the purity was 99.6 area %.Neither triphenylphosphine nor triphenylphosphine oxide was detected(less than 0.01% by HPLC-3).

To 21 g of the aqueous solution, concentrated hydrochloric acid wasadded at room temperature to adjust the pH to 2.8. Precipitated crystalswere filtered, washed with water, and vacuum-dried to obtain 5.3 g ofD-5-mercaptomethyl-5-methylhydantoin as a white solid. As a result ofHPLC analysis (HPLC-2), the purity of theD-5-mercaptomethyl-5-methylhydantoin of the resulting crystals was 99.6area %.

¹H NMR (300 MHz, NaOD/D₂O) δ: 2.69 (s, 2H), 1.33 (s, 3H)

[α]^(D) ₂₀=+98.26° (c0.50, 1N NaOH)

Example 8 Preparation of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid) (IodineOxidation)

In 72 g of water, 30 g of L-2-amino-3-mercapto-2-methylpropionic acidhydrochloride obtained by the same method as Reference Example 9 wasdissolved, and 26.1 g of 30 wt % aqueous sodium hydroxide solution wasadded thereto while maintaining the inner temperature at 25° C. or less.While maintaining the inner temperature to 20° C. to 25° C., 27.6 g ofiodine was added in three divided portions over 30 minutes undervigorous stirring, and the reaction was further conducted for one hour.The resulting reaction solution was analyzed by HPLC (HPLC-2). As aresult of the analysis, 0.6% of residualL-2-amino-3-mercapto-2-methylpropionic acid was detected.

The reaction solution was neutralized with concentrated hydrochloricacid at room temperature to adjust the pH to 2.8. The precipitatedcrystals were filtered, washed with water, and put under vacuum toobtain 23.0 g of (2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid)as a white solid.

¹H NMR (300 MHz, NaOD/D₂O) δ: 3.31 (d, 1H), 2.92 (d, 1H), 1.30 (s, 3H)

[α]^(D) ₂₀=+217.53° (c1.00, 1N HCl)

Example 9 Preparation of L-2-amino-3-mercapto-2-methylpropionic Acid byReduction of (2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid)

To 5.0 g of (2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid)obtained in Example 8 and 6.4 g of triphenylphosphine, 50 g oftetrahydrofuran and 6.4 g of concentrated hydrochloric acid weresequentially added at room temperature under nitrogen stream. Reactionwas conducted for 4 hours at 60° C. to obtain 67 g of a reactionsolution containing 6.3 g of L-2-amino-3-mercapto-2-methylpropionic acidhydrochloride. The residual ratio (by HPLC-3) of the(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid) in the resultingreaction solution was 0.3%.

Example 10 Preparation of L-2-amino-3-mercapto-2-methylpropionic Acid byReduction of (2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid)

To 5.0 g of (2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid)obtained in Example 8 and 6.4 g of triphenylphosphine, 50 g of tolueneand 6.4 g of concentrated hydrochloric acid were sequentially added atroom temperature under nitrogen stream. The resulting mixture was heatedto 80° C. A total of 7.75 g of water was added in two divided portionsto completely dissolve crystals of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid). Subsequently,the reaction was conducted for 24 hours to obtain 67 g of reactionsolution containing 5.9 g of L-2-amino-3-mercapto-2-methylpropionic acidhydrochloride. The residual ratio (by HPLC-3) of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid) in the reactionsolution was 0.3%.

An aqueous solution obtained by separating the toluene layer from thereaction solution was washed with 50 g of toluene four times. As aresult, neither triphenylphosphine nor triphenyl phosphine oxide wasdetected (less than 0.01% by HPLC-3). An aqueous solution ofL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride having apurity of 99.6 area % (HPLC-2) was thereby obtained.

The aqueous solution was concentrated to 13 g (degree of vacuum: 30 to60 mmHg, temperature: 45° C.), combined with 40 g of toluene, andsubjected to vacuum concentration (degree of vacuum: 40 to 60 mmHg,temperature: 40° C., distillation rate: 107 L/h·m²) to bring the totalweight to 15 g. Toluene (40 g) was added thereto and the resultingmixture was concentrated. The same operation was carried out a total offive times, and then toluene was added to obtain 184 g of a tolueneslurry of L-2-amino-3-mercapto-2-methylpropionic acid hydrochloride.Crystals were filtered, washed with toluene, and vacuum-dried to obtain6.2 g of L-2-amino-3-mercapto-2-methylpropionic acid hydrochloride as awhite solid. As a result of the HPLC analysis (HPLC-4), the purity ofthe crystals of L-2-amino-3-mercapto-2-methylpropionic acidhydrochloride was 99.6 area %, and the content of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid) was 0.3 area %.

¹H NMR (300 MHz, D₂O) δ: 3.18 (d, 1H), 2.89 (d, 1H), 1.60 (s, 3H)

Example 11 Preparation of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid) (OxygenOxidation)

In 100 g of water, 10 g of L-2-amino-3-mercapto-2-methylpropionic acidhydrochloride obtained in Reference Example 9 was dissolved, and 12 g of30 wt % aqueous sodium hydroxide solution was added while maintainingthe inner temperature to 25° C. or less. The solution was then allowedto react under vigorous stirring under air bubbling while maintainingthe inner temperature at 20° C. to 25° C. Three days were required tocomplete the reaction. The reaction solution was neutralized with 10 gof concentrated hydrochloric acid at room temperature to adjust the pHto 2.8. Crystals precipitated were filtered, washed with water, andvacuum-dried to obtain 6.6 g of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid) as a whitesolid. As a result of HPLC analysis (HPLC-2), theL-2-amino-3-mercapto-2-methylpropionic acid content was 0.3%.

¹H NMR (300 MHz, NaOD/D₂O) δ: 3.31 (d, 1H), 2.92 (d, 1H), 1.30 (s, 3H)

Example 12 Preparation of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid) (Iron-CatalyzedOxygen Oxidation)

In 30 g of water, 10 g of L-2-amino-3-mercapto-2-methylpropionic acidhydrochloride obtained in Reference Example 9 and 1 mg (corresponding to0.3 mg of iron) of iron trichloride were dissolved. To the solution, 9 gof 30 wt % aqueous sodium hydroxide solution was added while maintainingthe inner temperature at 25° C. or less. Reaction was conducted undervigorous stirring with air bubbling while maintaining the innertemperature to 20° C. to 25° C. It required 30 hours to complete thereaction. With the progress of the reaction, products were precipitated,and the reaction solution thereby formed slurry having a pH of 9.4.

To this reaction solution, 30 wt % aqueous sodium hydroxide solution wasadded to adjust the pH to 11.0. The precipitated products were dissolvedas a result. The resulting aqueous solution (51 g) contained 7.6 g of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid). Furthermore,trace amounts of reddish-brown crystals remained undissolved.

The iron content in the aqueous solution was 12 ppm (corresponding to0.6 mg). L-2-Amino-3-mercapto-2-methylpropionic acid was not detected(less than 0.1% by HPLC-2).

Example 13 Removing Iron from(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid) (Treatment withActivated Charcoal as Filtering Aid)

To 10 g (containing 0.1 mg of iron) of the aqueous solution obtained inExample 12, 100 mg of activated charcoal (produced by TakedaPharmaceutical Company Limited: activated charcoal Shirasagi A)preliminarily washed with 0.01 M aqueous sodium hydroxide solution wasadded, and the mixture was filtered through a filter paper (pore size:0.8 μm, composed of cellulose) and washed with 1 ml of 0.01 M aqueoussodium hydroxide solution to obtain 11 g of a clear liquid. The ironcontent in the clear liquid was 0.1 ppm (corresponding to 0.001 mg ofiron).

At room temperature, 10 g of the clear liquid was neutralized withconcentrated hydrochloric acid to adjust the pH to 1.8. Crystalsprecipitated were filtered, washed with water, and vacuum-dried toobtain 1.3 g of (2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid)as a white solid. The iron content in the crystals was 0.8 ppm(corresponding to 0.001 mg).

¹H NMR (300MHz, NaOD/D₂O) δ: 3.15 (d, 1H), 3.05 (d, 1H), 1.35 (s, 3H)

¹³C NMR (300 MHz, NaOD/D₂O) δ: 196.9, 176.2, 68.3, 50.2, 25.6

[α]^(D) ₂₀=+216.43° (c1.04, 1N HCl)

Example 14 Removing Iron from(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid) (Treatment withCellulose as Filtering Aid)

To 10 g (containing 0.1 mg of iron) of the aqueous solution obtained inExample 12, 100 mg of powdered cellulose (produced by Nippon PaperChemicals, KC Flock) preliminarily washed with 0.01 M aqueous sodiumhydroxide solution was added, and the resulting mixture was filteredthrough a filter paper (pore size: 0.8 μm, composed of cellulose) andwashed with 1 ml of 0.01 M aqueous sodium hydroxide solution to obtain11 g of clear liquid. The iron content in the clear liquid was 0.1 ppm(corresponding to 0.001 mg of iron).

Example 15/Comparative Example 1 Removing Iron from(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid) (FilterTreatment)

10 g of the aqueous solution (containing 0.1 mg of iron) obtained inExample 12 was filtered through a membrane filter (pore size: 0.45 μm,composed of cellulose acetate) and washed with 1 ml of 0.01 M aqueoussodium hydroxide solution to obtain 11 g of clear liquid. The ironcontent in the clear liquid was 0.1 ppm (corresponding to 0.001 mg ofiron) (Example 15).

Meanwhile, 10 g of the aqueous solution (containing 0.1 mg of iron)obtained in Example 11 was filtered through a filter paper (pore size: 4μm, composed of cellulose), and reddish-brown crystals passed throughthe filter. The filter paper was further washed with 1 ml of 0.01 Maqueous sodium hydroxide solution to obtain 11 g of turbid liquid. Theiron content in the turbid was 12 ppm (corresponding to 0.1 mg of iron)(Comparative Example 1).

Example 16 Crystallization of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid)

10 g of the aqueous solution (containing 0.1 mg of iron) obtained inExample 12 was neutralized with concentrated hydrochloric acid to adjustthe pH to 1.8, and the precipitated crystals were filtered, washed withwater, and vacuum-dried to obtain 1.3 g of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid) as a whitesolid. The iron content in the resulting wet crystals was 2 ppm(corresponding to 0.05 mg of iron).

Reference Example 10 Preparation ofL-2-amino-3-mercapto-2-methylpropionic Acid Hydrochloride

The standard of the iron content in the industrial-grade concentratedhydrochloric acid is 20 ppm or less. Usually industrial-gradeconcentrated hydrochloric acid with an iron content of 0.2 ppm or lessis available. The reference example described below concerns an exampleof producing L-2-amino-3-mercapto-2-methylpropionic acid hydrochlorideby using concentrated hydrochloric acid having high iron content.

To 7.8 g of L-2-amino-3-tert-butylthio-2-methylpropionic acid obtainedby the process of Reference Example 8, 64.4 g of concentratedhydrochloric acid (iron content: 4 ppm, corresponding to 0.3 mg) wasadded, and the resulting mixture was refluxed for 24 hours to obtain 70g of an aqueous solution of L-2-amino-3-mercapto-2-methylpropionic acidhydrochloride. The iron content in the resulting reaction solution was 4ppm (corresponding to 0.3 mg).

This aqueous solution (69 g) was concentrated to 13 g (degree of vacuum:30 to 60 mmHg, temperature: 45° C.), combined with 41 g of toluene, andsubjected to vacuum concentration (degree of vacuum: 40 to 60 mmHg,temperature: 40° C.) to bring the total weight to 22 g. To this, 41 g oftoluene was added and the resulting solution was concentrated. The sameoperation was carried out a total of six times to obtain 21 g of atoluene slurry of L-2-amino-3-mercapto-2-methylpropionic acidhydrochloride. Crystals were filtered, washed with toluene, andvacuum-dried to obtain 6.4 g of L-2-amino-3-mercapto-2-methylpropionicacid hydrochloride as a white solid.

The iron content in the resulting crystals was 52 ppm (corresponding to0.3 mg), and the content of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid) was 0.7%(HPLC-2). It should be noted here thatL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride tends to showdegraded stability with higher iron content.

Example 17 Preparation of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid)

In 15 g of water, 5 g (containing 0.3 mg of iron) ofL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride obtained inReference Example 10 was dissolved, and 4.5 g of 30 wt % aqueous sodiumhydroxide solution was added thereto while maintaining the innertemperature at 25° C. or less. The reaction was carried out for 60 hoursunder vigorous stirring with air supplied to the gas phase whilemaintaining the inner temperature at 20° C. to 25° C. The resultingreaction solution was subjected to HPLC analysis (HPLC-2), and as aresult, 6.3% of residual L-2-amino-3-mercapto-2-methylpropionic acid wasdetected.

Subsequently, while air is blown into the liquid, reaction was conductedfor 24 hours under vigorous stirring. The resulting reaction solutionhad a pH of 13.3 and a very trace amount of reddish-brown crystals wereprecipitated. As a result of HPLC analysis (HPLC-2),L-2-amino-3-mercapto-2-methylpropionic acid was not detected (less than0.1%).

The reaction solution was neutralized with concentrated hydrochloricacid to adjust the pH to 11.2 and aged overnight under vigorousstirring. Reddish-brown crystals were precipitated as a result.

The resulting aqueous solution (22 g) contained 3.7 g of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid). Moreover, theiron content in the resulting aqueous solution was 14 ppm (correspondingto 0.3 mg).

Example 18 Removing Iron from(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid) (Treatment withActivated Charcoal as Filtering Aid)

10 g of the aqueous solution (containing 0.14 mg of iron) obtained inExample 17 was filtered through 100 mg of activated charcoal (producedby Takeda Pharmaceutical Company Limited: activated charcoal ShirasagiA) preliminarily washed with 0.01 M aqueous sodium hydroxide solution,and further washed with 1 ml of 0.01 M aqueous sodium hydroxide solutionto obtain 11 g of clear liquid. The iron content in the clear liquid was0.9 ppm (corresponding to 0.01 mg of iron).

Example 19 Removing Iron from(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid) (Treatment withCellulose as Filtering Aid)

10 g of the aqueous solution (10 g, containing 0.14 mg of iron) obtainedin Example 17 was filtered through 100 mg of powdered cellulose(produced by Nippon Paper Chemicals, KC Flock) preliminarily washed with0.01 Maqueous sodium hydroxide solution, and further washed with 1 ml of0.01 M aqueous sodium hydroxide solution to obtain 11 g of clear liquid.The iron content in the clear liquid was 3.8 ppm (corresponding to 0.04mg of iron)

Example 20 Preparation of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid)

In 313 g of water, 105 g of wet crystals ofL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride obtained as inReference Example 10 (the wet crystals containing 88 g ofL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride in terms ofnet content, 19 mg of iron, and 16 g of toluene) was dissolved, and thesolution was concentrated under vacuum to 206 g. The tolueneconcentration in the solution was 28 ppm. Next, to the solutioncontrolled to 25° C., 228 g of water and 170 g of 30 wt % aqueous sodiumhydroxide solution were added over 30 minutes. The temperature of thesolution increased to 35° C., and the pH of the solution reached 9.5.The reaction was then conducted for 12 hours under vigorous stirringwith air bubbling in the liquid while maintaining the inner temperatureat 35° C. to 40° C. The inner temperature was then cooled to 20° C. to25° C., and the reaction was further continued for 12 hours whilemaintaining this temperature. The resulting reaction solution (584 g)had a pH of 11.2 and reddish-brown crystals precipitated therein.L-2-Amino-3-mercapto-2-methylpropionic acid was not detected (less than0.1% by HPLC-2).

The resulting reaction solution (281 g) was filtered through 2.07 g ofactivated charcoal (produced by Takeda Pharmaceutical Company Limited:activated charcoal Shirasagi A) preliminarily washed with 0.01 M aqueoussodium hydroxide solution, and further washed with 10 ml of 0.01 Maqueous sodium hydroxide solution to obtain 284 g of filtrate. Then, 280g of the filtrate was neutralized with 31 g of concentrated hydrochloricacid at room temperature to adjust the pH to 4.5, and the precipitatedcrystals were filtered and washed with water to obtain 47.6 g of wetcrystals containing 38.9 g of (2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid). The iron content in thewet crystals was 2 ppm (corresponding to 0.1 mg of iron).

Example 21 Preparation of L-2-amino-3-mercapto-2-methylpropionic Acid byReduction of (2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic Acid)

To 23.2 g of wet crystals of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid) obtained inExample 20 (wet crystals containing 19.0 g of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid) and 0.04 mg ofiron) and 24 g of triphenylphosphine, 38 g of water, 19 g ofconcentrated hydrochloric acid, and 190 g of toluene were sequentiallyadded at room temperature under nitrogen stream. The reaction was thenconducted for 20 hours at 80° C. The residual ratio of(2R,2′R)-3,3′-dithiobis(2-amino-2-methylpropionic acid) in the resultingreaction solution was 0.1% (HPLC-3).

An aqueous solution obtained by separating the toluene layer from thereaction solution was washed with 190 g of toluene four times. As aresult, neither triphenylphosphine nor triphenylphosphine oxide wasdetected in the aqueous solution (less than 0.01% by HPLC-3). An aqueoussolution of L-2-amino-3-mercapto-2-methylpropionic acid hydrochloridehaving a purity of 99.9 area % (HPLC-2) was obtained thereby.

The aqueous solution was concentrated to 45 g (degree of vacuum: 30 to60 mmHg, temperature: 45° C.), combined with 72 g of toluene, andsubjected to vacuum concentration (degree of vacuum: 40 to 60 mmHg,temperature: 40° C., distillation rate: 107 L/h·m²) to bring the totalweight to 60 g. The resulting solution was combined with 72 g of tolueneand concentrated. The same operation was carried out a total of fivetimes. Subsequently, toluene was added thereto to obtain 184 g of atoluene slurry of L-2-amino-3-mercapto-2-methylpropionic acidhydrochloride. Crystals were filtered, washed with toluene, andvacuum-dried to obtain 21.7 g of L-2-amino-3-mercapto-2-methylpropionicacid hydrochloride as a white solid. The purity of the crystals of theL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride was 99.9 area% (HPLC-4), the iron content was 2 ppm (corresponding to 0.04 mg ofiron), and the sodium content was 0.2 wt %. It was confirmed that theL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride was morestable than L-2-amino-3-mercapto-2-methylpropionic acid hydrochlorideobtained in Reference Example 10.

Comparative Example 2 Preparation of2-amino-3-mercapto-2-methylpropionic Acid by Reduction of2-amino-3-benzylthio-2-methylpropionic Acid

To 2,000 mL of liquid ammonia cooled to −78° C., 11.3 g of2-amino-3-benzylthio-2-methylpropionic acid was added, and 4.0 g ofmetallic sodium was slowly added over one hour. The resulting mixturewas then warmed to −33° C. and reacted for 1 hour, and ammonia was thendistilled off by heating to room temperature. The resulting reactionsolution was combined with 500 mL of deaerated water to conduct furtherconcentration. Concentrated hydrochloric acid (25 g) was then added toobtain 82 g of an aqueous solution.

As a result of HPLC analysis (HPLC-2), the aqueous solution contained8.0 g of L-2-amino-3-mercapto-2-methylpropionic acid hydrochloride andhad a purity of 96.2 area %. Furthermore,2-amino-3-benzylthio-2-methylpropionic acid was not detected (less than0.1% by HPLC-1).

The aqueous solution (80 g) was concentrated to 16 g (degree of vacuum:30 to 60 mmHg, temperature: 45° C.), combined with 50 g of toluene, andsubjected to vacuum concentration (degree of vacuum: 40 to 60 mmHg,temperature: 40° C., distillation rate: 107 L/h·m²) to bring the totalweight to 27 g. Toluene (50 g) was added thereto and the resultingmixture was concentrated. The same operation was carried out a total offive times, and then toluene was added to obtain 25 g of a tolueneslurry of L-2-amino-3-mercapto-2-methylpropionic acid hydrochloride.Crystals were filtered, washed with toluene, and vacuum-dried to obtain11.8 g of L-2-amino-3-mercapto-2-methylpropionic acid hydrochloride as awhite solid.

As a result of the HPLC analysis (HPLC-4), the content ofL-2-amino-3-mercapto-2-methylpropionic acid hydrochloride in thecrystals was 66 wt %, and the purity was 95.9 area %. The sodium contentwas 13 wt %.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an IR spectrum of(5S,5′S)-5,5′-[dithiobis(methylene)]bis(5-methylhydantoin) obtained inExample 2.

INDUSTRIAL APPLICABILITY

According to the present invention, in the production of an opticallyactive R or S isomer of 2-amino-3-mercapto-2-methylpropionic acidderivative or salt thereof useful as an intermediate for pharmaceuticalsand the like, a novel intermediate that can highly suppresscontamination by various impurities can be obtained. By using this novelintermediate, it becomes possible to easily and efficiently produce ahigh purity optically active R or S isomer of a2-amino-3-mercapto-2-methylpropionic acid derivative or salt thereof onan industrial production scale.

1. A process of producing an optically active2-amino-3-mercapto-2-methylpropionic acid derivative represented bygeneral formula (1):

(wherein Y¹ is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z¹ is a substituted or unsubstituted aminogroup, or Y¹ and Z¹ together form a divalent group; and * is anasymmetric carbon) or salt thereof, the process comprising the steps ofreducing an optically active 3,3′-dithiobis(2-amino-2-methylpropionicacid) derivative represented by general formula (2):

(wherein Y² and Z² respectively may be the same as or different from Y¹and Z¹ above; Y² is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z² is a substituted or unsubstituted aminogroup, or Y² and Z² together form a divalent group; and * is anasymmetric carbon) or salt thereof so as to cleave the sulfur-sulfurbond; and converting Y² to Y¹ and/or Z² to Z¹ as necessary.
 2. Theproduction process according to claim 1, wherein one of metal hydridereagents, alkali metals, metals and acids, alkali metal sulfides, andphosphine compounds is used as a reductant in the reductive reaction. 3.The production process according to claim 1, wherein the reductivereaction is conducted in the presence of an acidic substance.
 4. Aprocess for producing an optically active2-amino-3-mercapto-2-methylpropionic acid derivative represented by saidgeneral formula(1) or salt thereof, the process comprising a step ofremoving impurities in an aqueous medium solution containing theoptically active 2-amino-3-mercapto-2-methylpropionic acid derivative(1) or salt thereof produced by the process according to claim 1 into anorganic solvent phase immiscible with the aqueous medium so as to purifythe optically active 2-amino-3-mercapto-2-methylpropionic acidderivative (1) or salt thereof.
 5. The production process according toclaim 4, wherein the organic solvent immiscible with the aqueous mediumis an aromatic hydrocarbon solvent.
 6. The production process accordingto claim 4, wherein the pH of the aqueous medium solution is adjustedout of the range of 4 to
 5. 7. The production process according to claim4, wherein the pH of the aqueous medium solution is adjusted to 3 orless.
 8. The production process according to claim 1, wherein Y¹ is anunsubstituted hydroxyl group and Z¹ is an unsubstituted amino group. 9.The production process according to claim 1, wherein Y² is Y¹ and Z² isZ¹.
 10. The production process according to claim 1, wherein a compoundprepared by forming a sulfur-sulfur bond between two molecules of anoptically active 3-mercapto-2-methylpropionic acid derivativerepresented by general formula (3):

(wherein Y³ and Z³ may respectively be the same as or different from Y²and Z² above; Y³ is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z³ is a substituted or unsubstituted aminogroup, or Y³ and Z³ together form a divalent group; and * is anasymmetric carbon) or salt thereof and converting Y³ to Y² and/or Z³ toZ² as necessary is used as the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative represented bysaid formula (2) or salt thereof.
 11. The production process accordingto claim 10, wherein the sulfur-sulfur bond is formed by oxidation usingan oxidant.
 12. The production process according to claim 11, whereinthe oxidant is oxygen.
 13. The production process according to claim 12,wherein an ionic iron compound is used as an oxidation catalyst.
 14. Theproduction process according to claim 10, wherein Y³ is an unsubstitutedhydroxyl group and Z³ is an unsubstituted amino group.
 15. Theproduction process according to claim 10, wherein Y³ and Z³ togetherform ureylene group (—NHCONH—).
 16. The production process according toclaim 10, wherein Y² is an unsubstituted hydroxyl group and Z² is anunsubstituted amino group.
 17. The production process according to claim10, wherein Y³ is Y² and Z³ is Z².
 18. The production process accordingto claim 10, wherein Y³ is converted to Y² and/or Z³ is converted to Z²by hydrolysis after the formation of the sulfur-sulfur bond.
 19. Theproduction process according to claim 18, wherein the hydrolysis isconducted under acidic conditions.
 20. The production process accordingto claim 1, wherein the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative represented bysaid general formula (2) or salt thereof is contained in the opticallyactive 2-amino-3-mercapto-2-methylpropionic acid derivative representedby said general formula (1) or salt thereof.
 21. The production processaccording to claim 20, wherein the content of the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative represented bysaid general formula (2) or salt thereof in the optically active2-amino-3-mercapto-2-methylpropionic acid derivative represented by saidgeneral formula (1) or salt thereof is 0.1% or more.
 22. The productionprocess according to claim 10, wherein the optically active3-mercapto-2-methylpropionic acid derivative represented by said generalformula (3) or salt thereof is contained in the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative represented bysaid general formula (2) or salt thereof.
 23. The production processaccording to claim 22, wherein the content of the optically active3-mercapto-2-methylpropionic acid derivative represented by said generalformula (3) or salt thereof in the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative represented bysaid general formula (2) or salt thereof is 0.1% or more.
 24. A processfor producing an optically active 2-amino-3-mercapto-2-methylpropionicacid derivative represented by said general formula (1) or salt thereof,comprising performing crystallization in the presence of an organicsolvent from an aqueous medium solution containing an optically active2-amino-3-mercapto-2-methylpropionic acid derivative represented by saidgeneral formula (1) or salt thereof produced by the method according toclaim
 1. 25. The production process according to claim 24, wherein asalt with an acid of the optically active2-amino-3-mercapto-2-methylpropionic acid derivative (1) iscrystallized.
 26. The production process according to claim 24, whereinconcentration is conducted in the presence of the organic solvent toreplace water by the organic solvent while removing water from thesystem, and thereby crystallize the compound.
 27. The production processaccording to claim 24, wherein the organic solvent immiscible with theaqueous medium is used.
 28. The production process according to claim24, wherein the concentration and the solvent replacement are conducteduntil the amount of the residual water is 100 wt % or less relative tothe optically active 2-amino-3-mercapto-2-methylpropionic acidderivative (1) or salt thereof to thereby conduct crystallization. 29.The production process according to claim 26, wherein an evaporationrate during the concentration is controlled to 1,000 L/h·m² or less. 30.A process of producing an optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative represented bygeneral formula (2):

(wherein Y² is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z² is a substituted or unsubstituted aminogroup, or Y² and Z² together form a divalent group; and * is anasymmetric carbon) or salt thereof, the process comprising the steps ofoxidizing an optically active 3-mercapto-2-methylpropionic acidderivative represented by general formula (3):

(wherein Y³ and Z³ may respectively be the same as or different from Y²and Z² above; Y³ is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z³ is a substituted or unsubstituted aminogroup, or Y³ and Z³ together form a divalent group; and * is anasymmetric carbon) or salt thereof to form a sulfur-sulfur bond betweentwo molecules and converting Y³ to Y² and/or Z³ to Z² as necessary. 31.The production process according to claim 30, wherein the oxidation isoxygen oxidation.
 32. The production process according to claim 31,wherein an ionic iron compound is used as an oxidation catalyst.
 33. Theproduction process according to claim 30, wherein Y³ is an unsubstitutedhydroxyl group and Z³ is an unsubstituted amino group.
 34. Theproduction process according to claim 30, wherein Y³ and Z³ togetherform ureylene group (—NHCONH—).
 35. The production process according toclaim 30, wherein Y² is an unsubstituted hydroxyl group and Z² is anunsubstituted amino group.
 36. The production process according to claim30, wherein Y³ is Y² and Z³ is Z².
 37. The production process accordingto claim 30, wherein Y³ is converted to Y² and/or Z³ is converted to Z²by hydrolysis after the formation of the sulfur-sulfur bond.
 38. Theproduction process according to claim 37, wherein the hydrolysis isconducted under acidic conditions.
 39. The production process accordingto claim 30, wherein the optically active 3-mercapto-2-methylpropionicacid derivative represented by said general formula (3) or salt thereofis contained in the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative represented bysaid general formula (2) or salt thereof.
 40. A process for purifying anoptically active 3,3′-dithiobis(2-amino-2-methylpropionic acid)derivative represented by general formula (2):

(wherein Y² is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z² is a substituted or unsubstituted aminogroup, or Y² and Z² together form a divalent group; and * is anasymmetric carbon)or salt thereof, the process comprising adjusting anaqueous medium solution containing the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative (2) or saltthereof to be basic so as to separate and remove impurities from thesolution.
 41. The purification process according to claim 40, whereinthe pH of the aqueous medium solution adjusted to be basic is 10 ormore.
 42. The purification process according to claim 40, wherein theprecipitated impurities are removed by filtration.
 43. A process forpurifying an optically active 3,3′-dithiobis(2-amino-2-methylpropionicacid) derivative represented by general formula (2):

(wherein Y² is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z² is a substituted or unsubstituted aminogroup, or Y² and Z² together form a divalent group; and * is anasymmetric carbon), the process comprising adjusting an aqueous mediumsolution containing the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative (2) or saltthereof, to be neutral to acidic so as to crystallize the opticallyactive 3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative (2) andremove impurities.
 44. The purification process according to claim 43,wherein the pH of the aqueous medium solution adjusted to be neutral toacidic is 9 or less.
 45. A process for purifying a salt with an acid ofan optically active 3,3′-dithiobis(2-amino-2-methylpropionic acid)derivative represented by general formula (2):

(wherein Y² is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z² is a substituted or unsubstituted aminogroup, or Y² and Z² together form a divalent group; and * is anasymmetric carbon), the process comprising adjusting an aqueous mediumsolution containing the optically active3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative or saltthereof to be highly acidic so as to crystallize the salt with the acidof the optically active 3,3′-dithiobis(2-amino-2-methylpropionic acid)derivative (2) and remove impurities.
 46. The purification processaccording to claim 45, wherein the pH of the aqueous medium solutionadjusted to be highly acidic is 1 or less.
 47. The purification processaccording to claim 40, 43 or 45, wherein the impurities comprise aninorganic substance.
 48. The purification process according to claim 47,wherein the inorganic substance is an ionic compound of a heavy metal.49. A production process of producing an optically active2-amino-3-mercapto-2-methylpropionic acid derivative represented bygeneral formula (1):

(wherein Y¹ is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z¹ is a substituted or unsubstituted aminogroup, or Y¹ and Z¹ together form a divalent group; and * is anasymmetric carbon) or salt thereof, the process comprising the steps ofreducing an optically active 3,3′-dithiobis(2-amino-2-methylpropionicacid) derivative represented by general formula (2):

(wherein Y² and Z² respectively may be the same as or different from Y¹and Z¹ above; Y² is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z² is a substituted or unsubstituted aminogroup or Y² and Z² together form a divalent group; and * is anasymmetric carbon) or salt thereof so as to cleave the sulfur-sulfurbond; and converting Y² to Y¹ and/or Z² to Z¹ as necessary, wherein theoptically active 3,3′-dithiobis(2-amino-2-methylpropionic acid)derivative represented by said formula (2) or salt thereof Ls purifiedby the process according to claim 40, 43 or
 45. 50. A process ofproducing an optically active 3,3′-dithiobis(2-amino-2-methylpropionicacid) derivative represented by general formula (2):

(wherein Y² is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z² is a substituted or unsubstituted aminogroup, or Y² and Z² together form a divalent group; and * is anasymmetric carbon) or salt thereof, the process comprising the steps ofoxidizing an optically active 3-mercapto-2-methylpropionic acidderivative represented by general formula (3):

(wherein Y³ and Z³ may respectively be the same as or different from Y²and Z² above; Y³ is an unsubstituted hydroxyl group or a substituted orunsubstituted amino group and Z³ is a substituted or unsubstituted aminogroup, or Y³ and Z³ together form a divalent group, and * is anasymmetric carbon) or salt thereof to form a sulfur-sulfur bond betweentwo molecules and converting Y³ to Y² and/or Z³ to Z² as necessary andfurther comprising purifying the compound represented by said formula(2) produced by the above process by the purification process accordingto claim 40, 43 or
 45. 51. An optically active (2R,2′R) or(2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic acid) derivativerepresented by general formula (4):

(wherein —Y⁴-Z⁴- is a divalent group; and * is an asymmetric carbon) orsalt thereof.
 52. The optically active (2R,2′R) or(2S,2′S)-3,3′-dithiobis(2-amino-2-methylpropionic acid) derivative orsalt thereof according to claim 51, wherein —Y⁴-Z⁴- is a substituted orunsubstituted ureylene group (—NHCONH—).